Continuity mistake: In the scene where Brooks Hatlen is committing suicide you see him engraving "Brooks was here" into the wood panel near the ceiling. The table he is standing on puts his face opposite the vertical wooden bars underneath the engraving (we see this in a shot looking up at him through these bars). Then when he rocks to tip the table over to hang himself he doesn't seem to fall at all, maybe a few inches if anything, yet when we see the pan out he has fallen at least 2 feet.
Continuity mistake: The "A" that Andy carves initially is different from the "A" shown near the end of the movie showing how he escaped.
Continuity mistake: When Hank Williams is singing "Lovesick Blues" the MGM label is from 1968. This is meant to be 1963.
Continuity mistake: The warden pulls out a box of Remington .38 Special cartridges to load his revolver prior to his suicide. The warden empties the cartridges on to his desk and loads the revolver with what are Winchester/Western cartridges.
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.