First one on a camping holiday with a bunch of teenagers and 20 somethings. I really fancied this bloke but it turned out (after a lot of snogging) he was into my friend who wasn't on the holiday. So I slept with this nice guy on the rebound. In a tent! With no contraception. Years before MAP. Years before AIDS. Worried few weeks and one negative pregnancy test later and it was all fine.
One at uni, just been dumped after the exams by BF. Slept with his friend, sort of a revenge shag.
I was an idiot. Nice guy again but his only condom was in a key ring, sort of novelty item, bright blue and it split while putting it on so we just fumbled about a bit. Embarrassing next day/week/ third year when both sober.
Next one I brought back to my uni house after a party but despite lots of oral he couldn't get it up so I chucked him out as neither of us could sleep comfortably in my single bed. At 3am. Harsh.
Next one was a work colleague. We'd gone out in a group drinking and we both fancied a takeaway so he came back to mine. Great sex, but it turned out he'd just started dating someone else. No hard feelings but on my last day at that work he said, 'We must have a Chinese again sometime.' And I blushed really badly. 
Last one was the brother of my friend from school. We got something going at friend's housewarming party and dated for a couple of weeks, so not really a ONS, but only had sex once. Average. He was more in love with himself than with me. Met his family once.
6 months later went to another party at friend's house and got off with friend. Yes, last one's brother. His mum said, 'You again' when I went around there.
Reader, I married him.
22 years and 3 kids later now divorced.
No more ONSs since, sadly.