Crashing at my sisters place over Christmas break, and she didn't have a second key, so when I said I would be out of the house most of the next day, and that she would leave before me and be back before I was, she said that she would leave her key at home, and that I should drop it off at her work during the day so she could get in.
So I ended up coming back to the house during the afternoon, and while getting ready to head back out I made sure to leave the key on the table so that I wouldn't have it in my pocket downtown when my sister got off work
I was halfway down the street before I realized my error >.<
Fortunately, my sister was able to get ahold of her recently-former roommate, who hadn't gotten around to giving her key back yet, so all's well that ends well