When Vader enters Leia's cell for interrogation, a guard takes position to the right of the door. When the floating orb enters the room, suddenly it's a different actor standing there (lips and position of chin strap are different).
It is frequently claimed that upon returning to the Rebel base after destroying the Death Star, Luke exclaims, "Carrie!" when he hears Leia (Carrie Fisher) call "Luke!" Sound designer Ben Burtt has confirmed that, after extensive listening, it appears to be "Hey" or "Yay"; Mark Hamill reports that he said, "Hey! There she is!" but mumbled the last word. Yet, trained lip readers report only two syllables being spoken and they look like Carrie.
Back in the hangar bay on Yavin after the destruction of the Death Star, as Luke, Han and Leia are jubilant, C-3PO rushes to the side of Luke's X-wing. 'Anthony Daniels' in costume saying "Oh my! Artoo!" in the audio track. Shortly thereafter, C-3PO's proper "Oh my! Artoo!" (replete with robotic audio effect) is heard.
Just before Alderaan is destroyed, Governor Tarkin asks the Princess "You would prefer another target, a military target, then name the system!" In the reaction shot of the Princess you can see Tarkin's mouth moving but there is no sound.
During the heroes' procession into the award ceremony, the assembled rebel soldiers turn (either left-face or right-face, depending on which side of the aisle they were standing), accompanied by the appropriate sound of their feet stomping to complete the movement. However, only a few actors (visible on the left side of the aisle) actually stomp their feet after making their turn.
When C-3PO and R2-D2 are in the control room of the Death Star, the storm troopers barge in, and one hits his head on the door. This goof was highlighted in the remastered version with a comedy "donk" sound effect.
Luke Skywalker tells his aunt and uncle he wants to go to the "Academy", but the Academy is an Imperial institution - this makes little sense when he later specifically tells Obi-Wan he hates the Empire (Luke's motivation is hinted at in deleted scenes from the film, in which friends of him talk about joining the Academy and "jumping ship" to run off and join the Rebellion, but the remark is left making little sense in the final released film.)
When Luke is practicing with the lightsaber in the Millennium Falcon, his lightsaber is green. He turns it off to talk to Ben and when he turns it back on, it is back to its "proper" light blue. This mistake is only in the 2004 DVD release.
In the original theatrical release, when the laser bolt from the Death Star is fired at Alderaan, two separate explosions take place, not one, and some rubble that is fired from the planet during the first explosion disappears when the second explosion is taking place.
Even after Darth Vader's TIE's laser hits R2, he appears undamaged for the rest of the Battle of Yavin. He doesn't even appear damaged until the return to the base following the Death Star's destruction.
After C-3PO and R2-D2 escape from the Blockade Runner, the Blockade Runner has vanished from the Star Destroyer's underside tractor beam hatch. This is most evident in the last frame of the scene that shows the Star Destroyer flying by.
As Luke prepares to board his fighter prior to the climactic space battle, he brushes his hand along the underside of the wing. The markings on the wing clearly indicate it is "Red One," shown by the one red stripe. However, during the battle it is repeatedly established that Luke is in fact "Red Five," and exterior shots of the spacecraft model have five red stripes. This is corrected in the special edition by adding the scene of Luke talking to Biggs. They start talking under red one and continue to walk to Luke's fighter. This scene was cut from the original.
When the Falcon is escaping the Death Star, they are pursued by four fighter craft. After three of the fighters are destroyed, what should be the lone remaining craft is next seen accompanied by another. The second craft apparently reassembles itself just long enough for this one fly-by, as it is not seen again (this was corrected in the Special Edition, as only one TIE fighter is visible in the shot).
The blue panels on R2-D2 appear black when he is in the X-Wing with Luke in space. This is a blue screen shot which effectively makes R2's panels transparent. In the Special Edition, they left them as black but the newly added CGI X-Wing shots with R2-D2 are blue.
For the 1997 Special Edition, Industrial Light and Magic made CG models of the X-Wings (based on the "Red 2" filming model, Wedge's ship) and the Millennium Falcon (based on the filming model built for Star Wars: Episode V - The Empire Strikes Back). Thus in the new, CG Special Edition shots, Luke's X-Wing incorrectly has two red stripes on each wing (instead of five), and the Falcon has the two extra landing gear bays on its underside it didn't *originally* have until Empire (it also has a slightly different paint scheme).
When the Death Star is approaching the Yavin Base to superlaser it, the times to arrival at the laser firing point don't even come close to matching the ones on the clock on the monitor of the control room.
When Luke returns to the Lars homestead to find it destroyed, in all shots from behind Luke (or from his point of view) the wind is blowing from his left; all shots facing Luke show it blowing from his right.
Just after R5D4 "blows a fuse" when the droids are purchased from the Jawas and C3PO recommends R2D2, we see R5D4 behind R2 (as R2D2 stands against the sandcrawler). In the next shot, R5D4 is once again beside C3PO being taken back by the Jawas.
C3PO tells Luke Sywalker, as Luke is about to get his father's lightsaber from Ben, he is going to shut down if not needed anymore. His head movements change from shot to shot as he is exposed when Luke is playing with the lightsaber.
After their escape from the trash compactor, Han and Luke exchange their belts for stormtrooper belts and blasters. As they escape from the Death Star, Han sits in the pilot's chair wearing the stormtrooper belt, but when he gets up to go to the gunnery chair he's wearing his original gun belt and blaster.
When Luke and his uncle are talking at supper he takes the cup of blue milk away from his face and in the next shot when he asks, "He knew my father?" the cup is at his mouth again and he is again taking it away. Also, the cup continually switches from Luke's right hand to his left and back throughout the scene.
Just after Leia has been rescued from her cell on the Death Star and says sarcastically, "Great Rescue!" as they are being fired at, her fringe is smoothed away from her face. The next time we see her part of her fringe is falling across her forehead, and in the shot after that it is back in place.
After Leia blasts a hole into the garbage chamber and jumps in, Chewbacca begins to step into the hole. The shot cuts to Luke, but when it cuts back to Chewie he is once again beginning to step into the hole.
Direction of the shadows changes several times by almost 180 degrees between shots when R2D2 and C3PO are sold to Owen. Light conditions change simultaneously several times from bright to moderate and back.
In the shot directly before Owen approaches C-3PO, R5D4 is visible in the background in drive mode (body tilted, 3rd leg lowered). In the following shot, R5D4 stands straight with 3rd leg most likely retracted.
In the scene with the oil bath in Luke's garage, C3PO is holding a rag. We cut away to R2D2, then cut back to C3PO, and the rag is gone. Then cut to R2D2 again and cut back to C3PO, then the rag is back.
When Han Solo is arguing with Jabba the Hutt, Jabba yells: "Come on!" and jerks to the left. As he jerks, we see dark crescent-moon object: Declan Mulholland's head, not fully covered by the animated figure of Jabba that was superimposed later. Since Jabba was re-animated for the 2004 DVD release, that is the only time it is seen.
Considering what we now know from Star Wars 1-3, why doesn't Obi-Wan remember R2D2 and C3PO? We know C3PO's memory was wiped, so he wouldn't remember Obi-Wan, but Obi-Wan should remember R2D2, and vice-versa. However, there's no indication that he doesn't remember R2D2, he only says that he doesn't remember "owning a droid", which is actually true to some extent. (See also the FAQ entry about this issue.)
During the final Death Star battle, Red Leader says, "Stay there, I just lost my starboard engine" shortly before we see his fighter get hit. However, he may not have lost it due to a hit; it may have had a mechanical failure that just came at a really bad time.
After Greedo is killed in the Cantina we see him again crossing a street in the distance as Han and Luke are walking down the street. However, it's probably another alien of the same species, and they just look alike.
When Vader is in his TIE Fighter in the Battle of Yavin, more than once you can see the eyes and eyebrows of David Prowse through the visors on Vader's mask. This is not an error because Darth Vader has fully functional eyes, which could understandably be revealed through his mask. In this film the lenses of Vader's mask are a deep red color which is not visible through most of the film -- the lighting requirements in this scene are likely what made the lenses more transparent. In the subsequent films, the lenses were probably changed to a black color to compensate.
During the Death Star battle, you hear: "Red 6, can you see Red 5?" and a voice seems to answer. This is after Tono Porkins, who was Red 6, has been destroyed. But there is a hectic space battle going on with tons of Rebel and Imperial ships - and as a result, tons of radio calls. The response was likely an irrelevant call that just happened to get picked up at an odd moment.
When Obi-Wan talks to Luke about the force for the first time, there's no mention of "midichlorians" because they didn't exist back in 1977, when this movie first came out. It's clear George Lucas had a good overall idea of where he wanted his story to go, however it's also clear that many little details, including midichlorians, were added later on. Obi-Wan doesn't mention them here because Lucas hadn't thought them up yet. From the character's point of view, midichlorians are trivial and not significant enough to mention.
When telling Obi-Wan and Luke how fast the Millennium Falcon is, Han says it "made the Kessel run in under 12 parsecs." A parsec is not a measure of time. It is a measure of distance (approximately 3.26 light years or just over 19 trillion miles.) However, he was actually referring to the shorter route he was able to travel by skirting the nearby Maw black hole cluster, thus making the run in under the standard distance.
C3PO has been with Princess Leia's family since her childhood according to his back story established in later movies. During the skirmish at the beginning of the movie, C3PO says to R2D2 "there will be no escape for the Princess this time." (The script reads "There'll be no escape for the Captain this time," foreshadowing the next scene where Vader strangles the Captain.) Later, C3PO sees a picture of the Princess and claims not to know who she is. It's likely that it was programmed not to divulge any important information that may endanger the mission of the Princess, and the radio adaptation strongly implies this.
In the lightsaber fight between Vader and Obi Wan, David Prowse's eyes are visible through the Darth Vader helmet lenses. However, this is not an error because Darth Vader is a man with fully functional eyes, which could understandably be revealed through his mask.
Just before Leia and Luke swing across the chasm in the Death Star (and at other times), the "blasters" they are shooting are seen to eject shell casings. This is because the guns used in the movie are just dressed-up blank-firing prop guns.
When Obi-Wan activates his lightsaber to fight Vader, he is clearly just holding the saber so the long end points directly at the camera, and rotating it slowly to create the illusion of the blade appearing.
Most of the times, when Luke or Ben ignite or deactivate a lightsaber, the saber is not shown igniting or deactivating; just the sound effect his heard. And the times when it IS shown, even though it is supposed to be one shot, the character or his/her body part is in a slightly different location after the weapon is ignited than before the weapon is ignited, indicating two separate shots.
During a gun battle just before Luke and Han Solo escape down the garbage chute, some close-ups of Luke and Han show flesh-colored make-up smeared from under their chins onto the white plastic Imperial armor they are wearing. These makeup smears do not appear in longer shots, but show up in a couple of close shots during the scene.
After Vader strikes down Obi-Wan by the hangar door inside the Death Star, Vader's lightsaber is revealed only as a white stick. The lightsaber's red glow is gone and you can clearly see the white pole as the actors used it on the set. This mistake has been corrected for the CGI-enhanced special edition and DVD edition, but is still visible in the old VHS and Laser Disc versions.
(Special Edition) On the morning after R2's escape the image fades up to reveal the deserted courtyard of the homestead. Uncle Owen is heard calling for Luke who, as we find out a few seconds later, has already left with his Landspeeder in order to look for the missing droid. However, a close inspection of the courtyard shot reveals a ghost-like image of Luke in a doorway to the right. He seems frozen in mid-stride and has been slightly blended into the surrounding set, thereby turning somewhat transparent. This strange cameo is not featured in the original release.
During Kenobi's final lightsaber battle with Vader, Kenobi tips his lightsaber downwards, right before a scene change. The white "glow" that should be there isn't, showing the stick-like prop the actors used in filming. There is a small white light at the tip, too. This was not fixed in the Special Edition, but it was finally fixed on the 2004 DVD.
Leia blasts a hole in the wall to escape the detention level, leaving the remains of bars at the top of the hole. When Luke jumps down the garbage chute, his feet hit the bars lightly, making them shake, since they are made of foam.
The technical data that R2D2 uploads to the Rebels shows a schematic image of the Death Star with at the "laser divot" along the equator of the Death Star, not off-center like it was constructed. This is because the schematics were from different concept art of the Death Star not used in the final movie version.
After C-3PO and R2-D2 split up on Tatooine, C-3PO has an oil streak down his right side. When he comes across the large skeleton in the sand, the oil streak has switched to the left (film was flipped). When he is calling and waving to the Sandcrawler, the oil is back on the right side.
In the Special Edition, as a figure passes in front of Luke and Biggs in the rebel hangar, R2D2's position jumps as he is being lifted up to the top of the X-Wing fighter (right side of the frame). This is a result of editing out a section of the scene where Red Leader mentions having known Luke's father. The passing figure is intended to mask the edit, but the jump in R2's position reveals the edit. This is mainly evident in the widescreen format.
When Luke, Ben, and the droids are in the landspeeder approaching the stormtroopers at Mos Eisley just prior to the famous scene "These aren't the droids you're looking for" the starboard engine cover is clearly empty.
At the Mos Eisley Cantina, Luke gets into it with two criminals and gets thrown at a wall. Obi Wan Kenobi then turns on his lightsaber and proceeds to cut one of the criminal's arm off. Then there is a closeup of the severed arm and there is blood on the ground. Lightsabers usually "seal" the cuts they make allowing no blood to come out.
During the initial raid on the rebel ship there's a brief shot that shows the rebel troops with the small radio attachment on the right hand side of their helmets, where else it is on the left hand side (indicating flopped shot).
In the Special Edition, when Luke and Obi-Wan first enter Mos Eisley, the shadows (real and CGI) indicate the sun is near overhead. However, the large CGI beast that is passed has a shadow that indicates it is several hours later. Shadows appear consistent after that.
When Han puts his feet on the control board where Kenobi leaves them you can see that the boots of his storm trooper armor are really just normal leather work boots painted white.You can see the elastic fabric on the sides of the boots are still there.
During the Death Star battle, several times a space ship is seen "being locked on" in the sight device, whereupon the pilot starts to shoot. However, when the camera cuts to the outside, it always takes several salvos until the ship is finally hit.
Near the opening scene, when C-3P0 and R2-D2 cross the corridor barely missing fire from the stormtroopers, all portals of that type are not level with the deck. However RD-D2 rolls across as if they were level.