Continuity mistake: At the start when Andy opens a bottle of alcohol, he drinks it with the same hand he opened it with. His hand is shifted after the second time we see him take a drink. (00:03:00)
Suggested correction: Watch carefully he takes the cap off and palms it in his right hand and as he does, so he lowers his hand and grabs the glass bottle to have a drink.
While the mistake isn't very clear, this mistake seems to be about the second time we see him take a drink, after the cut. After he says it's inconvenient the gun was never found, he's seen lifting the bottle by the neck, then the next shot, he's holding it further down.
Continuity mistake: During the scene where Andy and Red meet for the first time (baseball scene) the sun is in Red's face and so, Andy shouldn't have the sun in his face. But in fact, sometimes Andy has the sun in his face and Red doesn't anymore. Either they move very quickly between scenes or (probably) the whole scene were filmed at least two times. (00:24:00)
Continuity mistake: When Andy is talking to Red for the first time in the yard, Red is playing catch with a few guys. Towards the end of the conversation Red catches the ball twice without throwing it back the first time. (00:25:00)
Visible crew/equipment: When Sgt. Hadley nearly throws Andy off the plate factory roof, the reflection of a boom mike is briefly visible in the shiny brim of Hadley's hat. Andy dropped the tar mop before approaching Hadley, and there are no other objects in this shot that could have caused the reflection. (00:35:00)
Continuity mistake: When the convicts are having beer on the roof you see Red taking a sip. The camera cuts to Heywood offering Andy a beer, and when it cuts back to Red taking the next sip his bottle is much more full than before. (00:36:55)
Continuity mistake: When one of the "sisters" gets beaten up by Capt. Hadley he tries to escape from his cell and holds on to the bars on the other side of the passage. From the side his right hand grabs a diagonal bar, but seen through the bars he is holding on to a vertical bar. (00:43:15)
Continuity mistake: In the scene where the warden is tossing cells and comes to "size up" Andy, it shows all of the chess pieces on the window being knocked over while one is in the guard's hands. After a short time one of the pieces is upright again, without anyone touching them. (00:48:30)
Continuity mistake: When Brooks has the knife to Heywood's throat, in one shot there is blood visible on his neck, yet in the next shot the blood is gone. (00:54:15)
Suggested correction: It was always there. But in the 3rd of 4 scenes it was significantly more visible as a bright red streak. The other 3 times it's a dark red dot near the knife point.
Continuity mistake: In the same scene where Brooks is hanging himself, he carves his name and he is almost eye level with the spot where he is carving his name. The place where the rope is tied is lower. How come when he finishes carving his name he stretches to put the rope on? It is lower than where he was carving his name. (01:00:25)
Continuity mistake: When Andy is playing the opera music, the whole prison stops what they're doing and stares at the speakers. When they show the infirmary, an African American man gets out of bed and looks out of the window. When the camera goes to the prison yard it pans around and goes right across the same African American man, who is now standing in the yard. (01:06:05)
Revealing mistake: When Tommy crumples up the test paper he had written for Andy and throws it into a waste basket the metal container shakes as if he had thrown something heavy. (Telling from the noise it makes the item must have been heavy.) (01:23:30)
Factual error: When the warden offers Tommy a Marlboro, the pack has Marlboro Miles on the side. Not in 1965 or '66. (01:30:00)
Continuity mistake: Andy is discovered missing. While the warden is walking to Andy's cell, he tells the guard to question "him", meaning the Morgan Freeman character. He points to Freeman with his left hand but when the scene changes its point of view he is pointing with his right hand. (01:45:45)
Plot hole: When the warden comes into the cell the morning after the escape, the poster covering the hole is fastened down on all four corners - impossible to do after squeezing into that small hole. [On the DVD commentary, the director confirms that this was a movie "cheat".] (01:49:05)
Suggested correction: It's a shame the director surrendered, but for someone as smart as Andy, this is the smallest of problems. He could've put weights on the bottom of the poster, or used magnets he embedded in the walls together with iron glued to the poster, or enough washers in the bottom to essentially do both.
If he can make a six foot escape tunnel, he can scrape out several small holes around the poster's edge, and insert the aforementioned magnets, glued in however he can manage (ie tape, chewing gum).
He could have simply used glue mixed with dirt to weigh it down. Once the glue dried it wouldn't matter how heavy the poster was, so long as there was contact.
How could Andy do that after he escaped through the hole in the wall?
I don't think it's what happened, but easily enough. Put magnets in the wall during his tunnel digging process - he had years - then something magnetic affixed to the corners of the poster. As soon as he was in the hole the poster would hang down and affix itself to the bottom corners.
Further, if the poster weren't "glued" on the bottom, it would not have been taught enough for the warden's pebble to go through it. If the poster were attached loosely, the pebble would have simply bounced off.
Continuity mistake: In the two different shots we have of Tim Robbins getting ready to put the book and folder into the safe for the last time (before he escapes) he collects the book and folder in two very different ways. First time he pulls the book towards the folder. Second time he brings the folder to the book. (01:49:55)
Factual error: Towards the end of the movie when Andy is escaping, he breaks open a sewage pipe and is covered with its contents. It is physically impossible for the sewage to shoot out of the break like it did. In order for that to happen there has to be pressure. But there is none there, because the end he crawls through is empty and the end is in the open air. (01:52:20)
Continuity mistake: When Andy is in the bank, in the background you can see a man in a "porkpie" hat and a white shirt move forward to talk to the cashier. As Andy stands up to leave, the man can be seen again, standing towards the back of the queue. (01:56:10)
Suggested correction: The man may have made an error to some paperwork (deposit slip or withdraw slip), and had to get another one and fix his mistake...which would have put him at the end of the queue. You don't get to jump back in line, unless offered that convenience.
Continuity mistake: When Norton loads his gun the bullets are scattered over the table. When he takes aim the bullets are neatly in one place, but after has has shot himself they are scattered as before. (01:56:45)
Continuity mistake: When the warden shoots himself, he places the gun on the middle of his neck but when we see him lying there the bullet hole is now on the left side. (This is corrected on the tenth anniversary edition. The director also confirms this change on the DVD commentary.) (01:57:00)
Continuity mistake: When Norton prepares for his suicide you see on the desk a photograph and, a bit away, a lamp. After he has shot himself the photograph is leaning against the lamp (it was too far away in the previous shot to have fallen against it). There's also some sort of card open on his desk on some paperwork which suddenly moves onto bare desk and gets shorter. (01:57:10)
