when you're hanging out with a long time friend whom you've either had some awkward sexual tension with for some time or haven't and one thing leads to another. You end up sleeping together...then decide it was weird and it will never happen again. You go back to being friends and never speak of it again. You decide it was just oops-friend-sex
"Yeah, it was super weird. We're definitely better off as friends. Just another case of oops-friend-sex I guess!"