I swear this happened once, but no one else seems to remember it, but years ago on a Monday night game I believe, a Bears QB kneeled in a game when he meant to spike it, and the game had to go to OT rather than having the time left to get a last-minute FG in regulation. The QB knew the mistake he made almost immediately and had a big goofy "whoops, I messed up" smile on his face.
I'll let you guess which former Bears QB that was, but the hint is he mostly wears khakis now.