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.
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.
Chosen answer: In the original novella it is revealed that Andy smuggled $500 into the prison inside of his rectum. During an interview in 2004 Robbins incorrectly quoted the amount as being $100. The narration up until Red's release is provided as Red writes his account of the events while still in prison, and employs the same method to smuggle the story out. But since the issue of Andy smuggling in $500 into the prison isn't addressed in the movie, we should assume that he smuggled it in. In addition to this, the wardens scams are described as "near slave labor." From this we can assume that it is possible the inmates are getting paid (an incredibly small) wage. Perhaps Andy, with his financial knowledge, knows how to haggle, barter and stretch a dollar. One last (but not as likely) scenario is that Red allows some sort of lay by system to inmates.