Stupidity: For a brilliant criminal mastermind, Cyrus makes so many idiotic mistakes. (1) He leaves his entire escape plan hidden in the wall for the guards to find later instead of trying to dispose or destroy them (2) He makes the fatal mistake of trusting Santiago completely and never suspecting a double-cross. At the very least, he should have checked all of Lerner airfield just in case the plane was hiding (which it was). (3) He never notices or even questions Billy Bedlam's sudden absence until he sees his body. Odd considering he notice Santiago's absence immediately. And (4) When escaping on the firetruck, he (pointlessly) chooses to ride on the ladder instead of the front seat where he stands out, and Poe and Larkin both spot him. No surprise his whole plan failed.
Stupidity: After Nick discovers the parole letter about Po, he crinkles it up and throws it down. After Po kills him, wouldn't you think the first thing he would do would be to pick up that letter and also take the letter that his daughter wrote and put them in his pocket so that no one else could discover them? (01:01:00)
Suggested correction: While I agree that this would have been a good idea, I've never thought of this in all the times I've watched this movie. Apparently, Cameron didn't either. The fact that it didn't cross his mind is a character decision, not a movie mistake.