Worst but most productive. My oldest, closest friend and I grew apart in our early 20s but wound up living in the same city a few years later. We go right back to being as tight as can be, still platonic but with some tension (helped along by friends and family). We were rarely single at the same time, so nothing happened other than some drunk cuddling.

Until one night she crashes at my place after a night out and we end up spooning, she turns back and asks me if I want to have sex. I'm drunk, she's attractive, I'm not seeing any downside.

Only I cannot get hard. I've never had whiskey dick and I'm not that drunk anyway. We're making out, she's stroking my flaccid dick, she moves down to try oral. It's a truly sad moment when someone you've known since 6th grade has your dick in her mouth and almost nothing is happening.

There's more stroking, she tries to get on top of my now semi-erect penis... but it just isn't working. She was kind of mad and there was no more spooning that night.

So it was awful sex - she feels unwanted, my dick has decided to rebel for the first time, etc. - but it cleared the question of what would happen with us right the fuck up. All sexual tension dissipated immediately.