Stupidity: Even though Hermione reminds them that they must not be seen after freeing Buckbeak, they fly toward the castle, whooping and cheering because they're flying. Then, they land in the courtyard of the castle, have a conversation, and then Sirius flies off with Buckbeak. Not the behavior of trying to remain unseen and unheard.
Stupidity: When Harry finds the Sword of Gryffindor at the bottom of the frozen pond, he breaks a man-sized hole in the ice, then strips down and jumps in. He could have broken a far larger hole in the ice, negating the risk of being unable to locate the hole upon coming back up. But because he doesn't, he nearly drowns when the locket resists and he can't surface (he couldn't have anticipated the locket's resistance, but that such a small hole would be hard to get out through was easily foreseeable.)