The poster is the least of the questions. How did Andy know there was going to be thunder that night? Where did that rock come from? How could he be sure to break the pipe? How did he know there was no grille at the end? How could he breath in there?
And, having made it out, how could he have had time to tidy himself up, walk to town and visit a dozen banks (and then get far enough away) before the alarm was raised? I assume prisoners would leave the cells sometime before banks open.
Sometimes you just have to ignore these details in order to enjoy the story. (Eg. it's years later and Andy is still working on that boat when Red finally arrives!)
Originally Posted by muggins14:
“If he let it hang and let gravity hold it back, as you say, then it wouldn't have been taut enough for the stone to have gone through the poster when the warden threw it.”
A poster that big, attached at two corners, and resting against the wall all around, would have inertia and drag. A sharp stone thrown hard enough would be more likely to go through, than to just displace it and bounce off.