Toward the end of Ricky's and Jane's first kiss in his bedroom the moving shadow of a crew member or piece of equipment can be seen at the bottom of window and blinds projected on the wall (to the left of the black floor lamp, behind Ricky, far left edge of frame}.
When Lester takes the bag of marijuana from Ricky, Ricky mentions that there is a card in the bag with his beeper number. No drug dealer with any common sense would leave any kind of evidence like this in a bag that could easily be discovered by police due to carelessness of the drug buyer.
When Ricky comes home and finds his parents watching TV, it is night and he is wearing his catering tuxedo even though he just came home from school (as he is carrying a backpack). (The scene - which wasn't in the original script - was moved from after the real estate party to earlier in the movie.)
Carolyn is wearing her gardening gloves in the garage as she lectures Lester about smoking and "setting a good example for our daughter." When she warns him that "you will not get away with this," she is not wearing them.
After Ricky Fitts's father has beaten him up for opening the cabinet containing his Nazi plate, he has no evidence of any facial scarring that would have been the result of the open wounds caused by the attack.
Lester driving to the song "American Woman" appears to be making a continuous right turn by the tilt of his steering while driving down a straight road (compare the position of the steering wheel in the next scene where he is actually driving the car into Mr. Smiley's). The Camry is obviously being towed on a process trailer for this shot but someone has neglected to straighten out the front wheels of the car before turning the engine off and securing it to the trailer.
When Lester hears that Angela is staying over for the night, he spills beer on the sink. In the next shot the beer is gone and the sink is clean. Also the beer bottle is held differently, showing the label from another side.
Carolyn is pulled to the side of the road in the Mercedes SUV in heavy rain, the windshield wipers are running in the wide shot, shut off in the dolly shot through the drivers door window on Carolyn and running again with the camera continuing the shot inside the passenger compartment.
When Carolyn (with squeegee) is outside cleaning the sliding glass patio doors opening out on to the pool for the open house a sheer curtain panel is visible inside, when she throws open the front doors to the open house and welcomes prospective buyers we see in the background the sheers are flanked by heavy plaid curtains drawn open to either side. After she fails to sell the house and closes up the patio door at the end of the (same) day the curtains and sheers have disappeared and she draws textured vertical blinds closed across the same glass doors.
Lester and Carolyn are already home in bed (cut from Lester's rose petal fantasy) when Angela pulls up in front of the Burnham residence in her car to drop Jane off (later same night post game). In the wide shot neither Lester's Toyota Camry or Carolyn's Mercedes SUV can be seen parked in the driveway.
In the scene where Lester tries to instigate sex with Carolyn, when the argument over Lester nearly spilling beer on the sofa ensues, Lester angrily stands up to throw a cushion at the sofa. It bounces off, clearly landing on the chair behind him. In the next shot, the cushion is nowhere to be seen, however on the following one of Lester, it is clearly visible on the chair.
When Lester wants to dial up Angela while his daughter is in the shower, you can hear that the first two digits he's dialing are the same and the next one is different. However, the phone book says that Angela's number starts with (yes, you guessed it) '555', so all three tones must be equal.
In the aerial/helicopter shots of Robin Hood Trail at the beginning and end of the film the large trees lining the street are winter bare. When Ricky and Jane encounter the funeral procession waking home from school together the trees are in full leaf.
In early scenes Lester rides to work in a carpool arrangement with his wife and his daughter. Reinforcing the sense he is a loser Carolyn drives (apparently her Mercedes SUV) and Jane rides in the front passenger seat relegating him to the lowest position of power in the back seat where he slouches. When he loses his job at the ad agency he drives himself home in a hitherto unseen Toyota Camry which is subsequently revealed to be his when he trades it in for a Pontiac Firebird.
When Carolyn opens the garage door to Lester working out, she opens it with her hand from the outside handle (manually) but it continues to open with an automatic garage door opener from the inside shot.
After Lester has been shot, Jane and Ricky Fitts come downstairs to the kitchen to investigate. In the first shot blood is shown dripping from the kitchen table from the POV of Jane and Ricky Fitts looking through a crack in the door. The next shot shows Jane and Ricky opening the door to see Lester dead at the table. The POV then returns back to the dripping blood through the crack in the door despite just showing the audience that Jane and Ricky have already opened the door and discovered Lester.
The white decorative pillow Lester picks up and throws off Carolyn's precious blue and white striped silk sofa bounces off the floor and lands in the armchair behind him, returns to it's original position against the arm of the sofa in the next shot and then reappears in the armchair when the camera cuts back to Lester..
When Ricky and his parents are looking toward the television, three framed pictures are visible on top. In the reverse shot from behind the television looking at the Fitts on the sofa the frames disappear, then reappear again in the Fitts' POV.
When Ricky Fitts walks from the house to Colonel Fitts washing his SUV, the garden hose is relatively straight. Later shots (Lester jogs up to and introduces himself to Ricky's father) reveal it to be serpentine.
When Lester first sees Colonel Fitts standing outside the closed garage in the rain, he is quite close to the garage door. But when Lester hits the button, the garage door swings out and Colonel Fitts is standing much farther away.
We see the reflection of the three joggers in the rear window of the Fitts' car because of the angle at which the camera is shooting the scene. Colonel Fitts appears to react to the reflection of the joggers, despite the fact that he would not be able to see it from the angle at which he's facing the rear window.
When Angela meets Lester for the second time (by the refrigerator) her head is touching the corner of the wall. In the reverse angle of Lester, her head is away from the wall (so the camera can see Lester).
Wooden arm chair in front of the window at Lester's end of the dining room table which appears in every other scene in the movie has disappeared so Jane can walk to that window unimpeded to look out and observe Ricky, the new neighbor boy she had just flipped off for videotaping her getting out of Angela's car and walking up to her front door.
When the rose petals begin falling from Angela's naked body on the Burnham's bedroom ceiling on to Lester in bed his head is centered on the pillow, as the camera goes tighter in subsequent shots his head shifts significantly camera right on the pillow.
When Jane exits Angela's car after the game Ricky Fitts stands up from a metal garden chair to turn the porch light on and continues videotaping Jane. The chair is turned on an angle toward the street below the (camera) left side jamb of the window. When Jane peeks out of her dining room window to see if Ricky is still there the chair is now perpendicular to the wall below the window shifted (camera) right of center.
A buffet/sideboard on the wall behind Carolyn's chair in several wide shots of the Burnham's dining room (from foyer and stairs) never appears in Lester's POV shots from his end of the table looking directly at Carolyn who should be sitting in front of that large piece of furniture against the wall.
The dining room table is set with 6 crystal/glass candle holders with white candles when Jane crosses the room to the window to see if Ricky Fitts is still watching her but during both dinner scenes/sequences with the family at the table we see only 4 holders when the candles are lit.
During the beginning of the film, after Jane gets onto the car, Carolyn is waiting by the driver seat with her arm on the windscreen. As Lester approaches the car, she moves her hand up, but the following shot shows her hand unmoved.
In the opening scene when Carole is cutting the American Beauty roses, she snips the first one on the stem, about 4 inches from the flower. A moment later when she is examining the flower, the stem is clearly a foot long or more.
Before Lester wakes up at the beginning of the movie, his slippers are pointing towards the bed and at an angle. When he gets out of bed, his slippers are in a position so that he can easily slip into them.
After he is beaten by his father for looking at the Nazi plate, Ricky Fitts looks in the mirror to examine his wounds. The edge of the camera is visible in the mirror for about a second, then the frame is rearranged so it is no longer visible.
In the opening sequence of the film we see an overhead shot of the Burnhams' master bedroom, Lester is lying in bed on his stomach, rousing when his alarm clock goes off. Circular white matte board cutouts cap the top of the lamp shades on either side of the bed blocking the bare light bulbs from shining straight into the camera lens (and probably hiding color correcting gels on the bulbs that would only be found in lamps on nightstands in bedrooms on stage).
When Lester is driving in his car after leaving his advertising job, the trailing motorcycle cop escorting the picture vehicle (Camry) being towed on the process trailer behind the camera car can be seen going in and out of the shot in the rear window.
When Lester is singing "American Woman" in his car, buildings from Burbank, California (including the building bearing "The Disney Channel" logo at the time) are clearly seen in the reflection of his windshield. The telephone area codes and newspapers set the film in the Chicago area.
The dancing 'Spartanettes' have a capital 'S' on their uniforms when it should be 'R' for 'Rockwell'; or 'RHS' for 'Rockwell High School'. Lettering in a high school sport the letter on the letter jacket is always the first letter of the name of the high school, not the first letter of the name of the mascot. The same is true of uniforms.
The "Nazi" plate is an obvious fake. It looks like the art department applied a swastika decal from a plastic model airplane kit to an unmarked plate of non-German origin. German porcelain of the era was extensively marked: hero "nazi" plate lacks manufacturer/factory mark, city mark, date of manufacture and the stylized Art Deco Third Reich "Eagle" acceptance mark (eagle clutching a wreath in its talons which surrounds the swastika). There are thousands of surviving authentic pieces from original period services floating around with correct markings, examples from major German porcelain makers including Rosenthal are readily available.
We see Ricky take out the cup of urine & explain to Lester that he needs clean urine to pass a drug test at home. The test obviously does not require the urine to be "fresh", so why doesn't Ricky just keep his own urine around from when he has not used drugs? Why does he need others'?
In the dinner scene where Lester throws the bowl of asparagus against the wall the picture frame has been centered between the windows over the console table, and the table lamp visible in an earlier scene has been removed. These changes are not down to continuity as such. The script originally prompted for Lester to throw the plate on the floor, however it was decided that he throw it against the wall instead without the knowledge of Annette Bening or Thora Birch. It had to be done in one take (which it was) to get a real shock reaction. The lamp was removed to avoid any debris knocking it over, and the picture was repositioned in-line with Lester's aim for a clean throw and to minimize any risk of Kevin Spacey knocking the picture off the wall spoiling the 'one-take shot'.
When Angela and Jane are outside the school talking to Ricky, there is a shot in which the filter of Angela's cigarette is visible, and it's clearly white. As the cigarette is half-smoked, it should be brown from the tar. However, this could be deliberate as it could be that Angela and Jane are not really smoking and just trying to look cool.
In Jane's phone book, she has Lester's work phone number as 555-1251; he gives it as 555-0199, but he may have multiple lines. However, this is not her mother's work number of 555-0195, despite some reports to the contrary.
After Carolyn and Buddy order takeout in Mr. Smiley's drive thru (Lester hears them ordering inside on a speaker) they then drive past the menu board with the intercom/speaker for placing orders (we clearly saw Lester order here in a earlier scene). We can see they have never actually stopped at the menu station to order or to be overheard ordering by Lester inside.
When we see Carolyn scrubbing the kitchen counter preparing for her open house, the kitchen sink and window are on the wall camera left. Later when she is showing the house to prospective buyers the kitchen sink and window are on the wall camera right from the same POV. The 180 degree change in the room layout is evidence that the negative has been flipped from one scene to another.
Ricky Fitts walks from the Fitts house to Colonel Fitts, who is washing his car, into a giant, rectangular shadow. That shadow is not possible through real-world causes, because the Fitts' house is simultaneously casting a shadow forward, towards the street.
Carolyn Burnham's business phone number for Burnham Realty is listed as 555-0195 on her yard sign, but 555-0184 on the one-page fliers for the open house in the Plexiglas stand on the welcome/info table.
The goof items below may give away important plot points.
Lester is shot in the back of his head, yet when Ricky and Jane walk in we see Lester's face, but there's no exit hole. The position of the gun and the fact that blood is splattered on the wall means there should be an exit hole in his forehead, but there isn't.
In the final sequence when Lester's life flashes before his eyes, there is a close up of the wing of his cousin's "brand new" Pontiac Firebird. The paint on the wing shown has a decidedly aged patina indicating that the car shown is by no means brand new.