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)
Continuity mistake: When Hank Williams is singing "Lovesick Blues" the MGM label is from 1968. This is meant to be 1963.
Revealing mistake: When Brooks is carving his name, "dust" seemingly like plaster dust vs wood dust is shown falling on the table top, his shoes etc. When they cut to the view of Brooks looking at his work and closing his knife, there is no "dust" on his hand or knife. That much carving with that much dust falling on the table would have left his knife covered with dust. It was clean.
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.
Other mistake: When the opera records are being played, the camera stops near a loudspeaker in the main yard. There are two terminals for the wiring of the speaker, but no wire is to be seen; however, music is heard coming from that speaker.
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.