Other mistake: When the cons are drinking beer on top of the roof, the man that finished his bottle gestures for another. The guy takes out an already-opened bottle of the bucket horizontally, the beer should've poured out.
Suggested correction: The bottle is only horizontal for a second as he lifts it; enough time for some of the contents to pour out but not all.
Other mistake: When the warden is questioning the night guard in Andy's cell the morning-after the escape, he rips the clipboard out of his hand and hands it to Hadley. The pencil is stuck to the paper but is not under the clip. The pencil would have flown off.
Suggested correction: I've checked this frame by frame. The eraser at the end of the pencil is under the clip. It's secured in position very well.