I had a seinfeld like experience once. A girlfriend I really didn't care about had a hot roommate. My friend and I saw the episode with the 'great switch' one day while stoned and decided it would be cool to see if it worked. I tried, she wanted the threesome. I freaked out a little.

It really wasn't a threesome though. She basically sat there fingering herself and yelling at us (mainly me) what to do. There was minimal interaction with her and she just watched me fuck her roommate for an hour.