I don't think I've heard any really bad ones. Mostly it's just been guys (and a couple of women) who were exaggerating (or totally making up) their exploits.
The "worst" I ever told was not admitting it when a friend asked me if I'd slept with his wife (before they started dating). I just couldn't see anything good coming out of his knowing I had slept with her, so I flatly denied it. They're still together and he and I are still friends, so I still feel pretty good about this one.