Revealing mistake: Near the end, when Michael is unscrewing the sink before kicking the wall in, watch his hands in the foreground while he is talking to Sucre. Just when Sucre says "But what you were doing up there in the infirmary." Michael starts using his fingers to take the bolt out. You can see that there is no bolt, just him turning air, with a mystery 'unscrewing' sound effect.
Revealing mistake: After Pope enters Scofield's cell to see the escape hole, we see him standing outside the cell looking out at everyone throwing toilet tissue etc. If you watch closely right after we see a guy pulling his pillows toward him to tear up, it cuts to another inmate who is throwing stuff out as well. Only thing is, we see the cell door slides to the side slightly which would indicate that the cell doors where not locked. In the event of an escape, lockdown would happen immediately, with all cell doors locked with the prisoners inside their cells.