Plot hole: When the warden comes into the cell the morning after the escape, the poster covering the hole is fastened down on all four corners - impossible to do after squeezing into that small hole. [On the DVD commentary, the director confirms that this was a movie "cheat".] (01:49:05)
Suggested correction: I just watched the scene and the top and bottom of the poster is never shown. It's possible that only the top was pinned to the wall.
Suggested correction: It's a shame the director surrendered, but for someone as smart as Andy, this is the smallest of problems. He could've put weights on the bottom of the poster, or used magnets he embedded in the walls together with iron glued to the poster, or enough washers in the bottom to essentially do both.
That's a load of rubbish. Embedding magnets in the wall? Really.
If he can make a six foot escape tunnel, he can scrape out several small holes around the poster's edge, and insert the aforementioned magnets, glued in however he can manage (ie tape, chewing gum).
He could have simply used glue mixed with dirt to weigh it down. Once the glue dried it wouldn't matter how heavy the poster was, so long as there was contact.
How could Andy do that after he escaped through the hole in the wall?
I don't think it's what happened, but easily enough. Put magnets in the wall during his tunnel digging process - he had years - then something magnetic affixed to the corners of the poster. As soon as he was in the hole the poster would hang down and affix itself to the bottom corners.
Further, if the poster weren't "glued" on the bottom, it would not have been taught enough for the warden's pebble to go through it. If the poster were attached loosely, the pebble would have simply bounced off.
