Humor. I've made more than enough embarrassing mistakes while teaching - most recently, I was flying through a quick synthetic division of a 5th degree polynomial, answering a question at the same time to a student, and added 1 + 1 and got something that wasn't 2. About 1 second later (i.e. I finished the problem), a student pointed it out. "I did that on purpose to see who was paying attention! Only one of you knows what 1+1 is?!" Of course, they all knew it was carelessness (and multitasking), but coupled with trying to look innocent about it, when I'm obviously guilty, and making that statement, it's more or less a joke within class.