Character mistake: You'd think prison guards on Death Row would have known a little more about safe restraints, the way they bind and gag Percy could eventually kill him. They stuff a handkerchief all the way into his mouth, then tape his mouth closed. The gag reflex could suck the handkerchief into his windpipe and suffocate him in about three or four minutes. Even if he might be able to find away around the danger of suffocation the fact remains that no trained, experienced prison guard is ever going to restrain someone in that way.
Suggested correction: Not everyone even has a gag reflex, and it is easy, though not comfortable to push the handkerchief forward into the mouth with the tongue. You can see Percy breathing through his nose, so he's not running out of air. The other guards had no intention of leaving him in the cell for a long time, only long enough to teach him a lesson.
It doesn't matter what he "might" be able to do, or even if he could push the handkerchief forward. That's not the mistake. A trained, experienced prison guard would not restrain someone like that. Never.
Again, what might or might not have happened is irrelevant. No skilled, trained, experienced prison guard would apply a gag in this way.
Character mistake: Henderson asks the guy as he's leaving the room to get him some bourbon. They already drank in the bar and spat it out when they found out that everything had turned to blood, so it would still be blood.