I occasionally try to flip down my helmet, only to realize that I'm not wearing one.
Though my personal dumbest was in my friends car...I pushed the cigarette lighter in (I don't smoke) and pulled it out and asked "Does it work?"
Without thinking I touched the hot end with my finger. A sizzle and the scent of burned flesh later, I had my answer. Damn welding fumes. (yeah, that's it!) ;)