I had just started dating this guy. We were at his place one night, waiting for some other friends to show up so we could have dinner. We had about an hour, and since we were in that new-relationship-perpetually-horny stage, he asked if I'd give him a blowjob while we waited.
Now. This guy was not a blowjob guy. During one of his very first blowjobs, the girl had bitten him, and there had been blood. It took years before the mention of 'blowjob' wouldn't make him entirely limp. It was even more before he could come from a blowjob, so I was figuring I'd go down on him for a while and then we'd fuck. So I start sucking.
He and I were into a bit of rough trade. Maybe that did it. He reached down at one point and clamped his hand around my neck. I got a little dizzy, but soldiered on. He moved his hands to my hair, and was setting the rhythm.
And then I tasted blood. I panicked, and had this series of thoughts:
"Oh, shit, I've ruined blowjobs for him for another ten years!!"
"Wait. He's still fucking my mouth. What does that mean?"
"Oh. It's not his blood."
I opened my eyes, and there was blood ALL OVER his dick, and his legs, and the bottom of his shirt. Presumably it was all over my face. I stiffened, and sat up. He made a little disappointed noise and then opened his eyes. It was all downhill from there.