Stupidity: Kyle Miller gets in a confrontational talk with the villains to convince them that they need him to cut the diamonds he has in the safe; Jonah sums it up as "he wants them to get the diamonds, then give them back to him." Problem is, Kyle does not have any diamonds in the safe, and he knows it. He is selling so hard the bargain on something he does not have. If they agreed, he could not show them any diamonds and he'd have an even harder time dealing with them, and he does not have anything else to give them. Of course if he told them he had no diamonds the movie would have been much shorter, but he didn't have in-character reasons to lie on something like that.
Stupidity: There is a scene about a null hypothesis proving Cassius is the killer. Nearly everything is wrong in this scene. Statistically, you never prove a null hypothesis true. (01:23:11 - 01:24:22)