I dated this chick who was a roommate of a girl i worked with. She wasn't particually attractive, but she'd do anything - and I mean anything...she even barked during sex. She was basically my "practice girl" - since I didn't care what she thought, I'd try different things, and experiment...you know...get my style down. (Yes, I was an asshole then.)
When I say they were roommates, they literally shared a bedroom, with the two beds in an L shape. One night, while I was with my GF biblically, her roommate was asleep in her bed. (This was weird the first couple of times, but I got used to it, because, well, I was 19 and getting some.) I don't know how I got the idea in my head or why I did it, but just as I was about to...finish...I crawled up the bed and...well, finished - all over the roommate's sleeping face.
The epilog was the best part: The next morning, she told me she woke up, face stuck to the pillow and knew what happened. She then smiled and told me that the next time I do that, to wake her up first.
Looking back, I don't know why I never messed around with her or even tried a threesome, since the door was obviously there...