The narrator says "Five hundred yards. The length of five football fields. Just shy of half a mile." There are 1760 yards in a mile. 880 yards is half a mile. 500 yards is not even a third of a mile, a long way from 'Just shy.'
When Tommy goes out into the yard to talk to Warden Norton, Warden Norton offers him a cigarette. The pack of Marlboros that he offers him has Marlboro Miles on them, which weren't around until the 1990s.
When they are on the roof during the tar job, in either 1949 or 1950, Andy tells Hadley that the IRS allows a gift to a spouse tax free. At that time, the IRS was known as the Bureau of Internal Revenue. It did not become known as the IRS until the 1950s.
Andy's 1955 library letter carries a 1954-series 3-cent stamp, which is correct; but the letter beneath his has a 1923 series, which hadn't been issued since 1938, and the other two have just two cents in postage.
When the inmates are at the table wondering if Andy Dufresne wants to commit suicide, one of them says "No, no, Andy would never do that." But his mouth movements correspond with Dufresne rather than Andy.
Andy Dufresne is obviously a very intelligent man and fond of playing chess. However if you take a good look at the shot of the (nearly) completed chess board in his cell, you see that he put up the chess board the wrong way. The board should be turned 90 degrees in order to have the pieces stand right. The square on the lower left should be black, the one on the upper right too, which they are not, they are white. Anyone fond of playing chess would never make that mistake.
A friend of the warden bribes him with a pie made by his wife which was baked in a disposable aluminum pan. The warden later gives the pie to Andy, who shares it with Red. While Red's eating it, the pan is now made of strong metal.
When Brooks delivers the rock hammer to Andy's cell, he continues to push the cart in the same direction and we hear wheels squeaking for a few seconds as the camera changes angle, to Andy's POV. Later, when the warden leaves Andy's cell, we can see the end of the aisle on the cells' tier just outside Andy's cell. Brooks had nowhere to push that cart.
When Red and Andy meet for the first time, an extra is seen behind each of them. It is the same scene but as the lines were filmed at different times, the extra is wearing different shirts - one prisoner shirt the day that Red's lines were shot, and a different shirt behind Andy the day his lines were filmed.
When Andy is assigned to the prison library, Brooks tells him he'll give him a tour. As he says this, Brooks is standing in the doorway to the library. The room is very dimly lit. But when Brooks and Andy walk in seconds later, it is full of sunlight.
In the aerial shot where Andy's bus approaches the prison, next to the gate is a red brick building and it's sidewalk is strewn with debris. As the bus pulls in seconds later and we see the gate and same brick building from the vantage point of guards who hurried from a tower, the sidewalk is clean.
When Red is talking to Andy for the first time, he is throwing a baseball between himself and Heywood. Just as Andy stoops down, Red catches the ball and seconds later, catches the ball again without throwing it back.
In the original scene where Andy is switching the books and shoes he hands the warden the deposit slips and says, "three deposits". When Red is retelling the story and the scene is repeated Andy says, "three deposits today."
When Andy Dufresne's cell is about to be searched, as the Warden and guards are first approaching the cell, the shot shifts to Andy sitting in his cell holding an almost new indigo blue Bible as if reading it. Throughout the scene Andy holds the Bible in his hand as the guards ransack the cell. At the end of the scene, the search complete, the Warden enters the cell and, facing Andy, notices him holding the Bible then asks him about his favorite passages. As the Warden and Andy discuss Mark 13:35, the Warden reaches for and Andy hands him a worn and stained, black Bible.
When Red is on the bus, one shot shows all of the windows on the bus open. In fact, Red's arm is hanging outside the window. In the very next shot of the bus (a long shot from a distance), all of the windows are closed.
When Andy is undressing in his cell, it is revealed that the suit and tie he took from Norton are solid black. But when he walks into the Maine National Bank the next morning, he is wearing a striped suit and a striped red/black tie.
Towards the beginning of the film, during a beautiful aerial shot of the bus entering Shawshank Prison, the camera flies over the buildings, where we see the prisoners on their way to "greet" in the new inmates. As the camera circles around, at the top of the frame, in an area of green grass, near a building, the shadow of the camera's helicopter is clearly visible.
Andy's library has received donations of 78-rpm albums, and he selects a record from such an album before he locks himself in and broadcasts Le Nozze di Figaro. The record he plays, however, is a 33 1/3 rpm, which is clear from the position of the needle on the disk.
We see from the warden looking into Andy's tunnel and later from Andy crawling through it that it is several feet long - at least 2-3 body lengths. Andy is in an upper level of the prison. All he has to tunnel through is the wall itself, which cannot possibly be this thick.
When Red is sitting in his chair in his apartment with the compass you can tell he has done the scene over and over again. When he opens the compass it does not spin. With that type of compass when it is closes it locks the compass needle in place. So when you open it it spins until it settles on North. When he opens it however it is pointing perfectly north and does not spin.
If you look at the prison buildings in the background of most outdoor shots, you'll notice many of the panes of glass are missing. It's most obvious when Andy is being dangled from the roof of the number plate factory and again in the scene where the prisoners are in the yard listening to the opera music.
When Red takes the bus down to Mexico after his release in 1967, the highway shows yellow solid and yellow dashed center markings. In 1967, the dashed markings would have been white. Dashed markings did not start to be repainted yellow until 1971-1974 when MUCTD introduced the new scheme.
Near the end as Red was on the Trailways bus, we are shown a shot of the bus going down the hill and up the other side. On the far right, there was a green bicycle sign on a telephone pole that probably didn't show up until the 90s.
Shortly after Andy's escape, Red receives a post card sent by Andy from Ft. Hancock, TX. Red turns the card over and you see it was addressed but there was no written message. In the upper right corner you see what is clearly the royal-purple 3-cent Statue of Liberty stamp. However, a postage-rate increase that took effect in January 1963 raised the domestic post card rate to 4 cents. A post card mailed in 1966 would likely have borne the maroon 4-cent stamp depicting Abraham Lincoln.
As Andy is driving toward his destination in Mexico he is in a mid-size 1968 Pontiac convertible, a LeMans, Tempest, or GTO. This model would not have been released until September 1967 at the earliest, yet we are told that Andy escaped in 1966.
When Brooks hangs himself by kicking the table away, his feet barely lower at all, yet in the next shot we see that he should have fallen a considerably greater distance. Also as he is carving his name in the beam, his head is high enough that he would not have to stand on his toes to put the rope around his neck, as he is shown doing.
When Andy is in the bank withdrawing the warden's money, we
can see a man standing at the counter talking to a teller. A moment later when Andy leaves, the same man is walking up to the teller's window.
The bullet hole under the warden's chin is in a different
location than where he placed the gun barrel a moment before he committed suicide. In the 10th Anniversary release, director Frank Darabont admits that this was an error, it has bugged him for 10 years, and they had it fixed in the 2004 release.
When Red's hat is blown off by the wind, it tumbles almost
into the water, and Red walks quite close to the sea. But the next time we see Red, the hat is farther from the sea than Red originally walked.
When Norton is about to load the revolver you can see a black and white photograph in a white frame on the desk. In the next scene when Norton has shot himself, you can see that the picture has now moved across the desk and is leaning against the lamp.
When Norton is walking to Andy's cell after his escape, he tells the guards to question "that friend of his... him!" and he points to Red with the index finger on his left hand. The shot then cuts to a frontal shot of Red in his cell, and Norton is now pointing at Red with the index finger on his right hand.
After Andy escapes and is celebrating with upraised arms, the clothes he had just taken off (as narrated by Red when he said, "...the only thing they found was a set of prison clothes...") are gone, when he had just stripped them off and dropped them right next to him only seconds before.
Right before Warden Norton shoots himself, the layout of the bullets on his desk changes between shots. You can see, right before he aligns the bullet chamber with the barrel, that the bullets are a little spread apart from each other, with one of them adjacent to the bottom of the magnifying glass handle. In the very next shot when the bullet chamber is aligned and the gun is ready to fire, the bullets are close together in a single pile, and the one near the magnifying glass has disappeared.
When Norton is at his desk loading the revolver the brand of ammunition shown on the box is Remington Kleenbore. However, looking at the cylinder as he is loading it, the cartridges have a WW head stamp indicating Winchester Western ammunition.
When Warden Norton shoots himself, the bullet exits the window just below the blinds, creating a big hole and leaving blood stains. In the next scene, you see the window again, but now the hole is inches below the blinds and is a lot smaller in size, with the blood stains in different spots.
When Andy's climbing out of the hole, just before he's going into the sewer, you see him with his old shoes. But just before that he had put them in the warden's shoebox and replaced them with the warden's. However, at that point, Andy had been in prison for nearly 20 years and enjoyed special privileges. It's possible he had an extra pair of prison issue shoes in his cell, and took the warden's off before making his escape. This makes sense considering that when Andy entered the bank the next day, the shoes were still polished, and clearly had not been worn while wading through a sewage pipe.
It's been claimed that when the warden throws the rock through Raquel Welch on the morning after Andy's escape, the poster appears to be stuck to the wall at all corners, raising the question of how Andy could have re-stuck the poster from inside the tunnel. However, as shown in a later (flashback) shot, the poster is attached at the top two corners allowing it to be lifted like a curtain while Andy works behind it. With the poster mounted in this fashion, the only requirement for the poster returning to the proper position - after Andy makes his escape - is simple gravity.
On the morning of Andy's escape, he had the Maine National Bank (with their outgoing mail) deliver a package to the Portland Daily Bugle. Next you see the headline "Corruption, Murder at Shawshank." There is no way that article could have been written, published, and delivered that same morning, seeing as the reporter probably wouldn't have gotten the package until the following day.
When the top of the pipe is broken open, it shoots a fountain, indicating that it is pressurized (if only by gravity). But then the pipe drains nearly empty. It should only have drained to the lowest point in the hole broken at the top.
When the warden opens Andy's Bible from the safe, he opens it to the bookmark in Exodus to find the cutout where Andy hid his rock hammer. Exodus is the second book in the Bible, so there'd only be a few pages (Genesis) above the cutout. But when the warden drops the Bible to the floor, it falls open to the bookmarked page. Now about half the pages are each side of the bookmark, with one half cut out and the other not.