In high school I had a good friend; let's call her Ally. There was a bit of weirdness because at one point my girlfriend cheated on me with Ally, but when I became single in college Ally and I started hanging out a lot again. After a few months, she asked me to come to her birthday party, which was pretty much just going to a show and all going out for early hangover-prevention breakfast afterwards. A few days before her birthday she told me that everyone else had bailed, so it was just the two of us. Past me assumed Ally was upset no one would be there, so I offered to invite this guy I'd been talking to. Ally seemed just fine, but when my guy and I got to the show, Ally was nowhere around. When she finally text me back, she said she'd decided to go out with another friend to a different thing, and to have fun with my guy. I did have fun with him, and a few years later I married him. However, that was more or less the last I ever heard from Ally. She kind of dropped off the face of the planet. A few months ago I was talking to a mutual friend, one who was supposed to be at Ally's birthday. Turns out all that hanging out we had been doing led Ally to believe we were girlfriends.

TL;DR: I invited a boy I wanted to date to a friend's birthday party and hooked up with him without knowing my friend thought she and I were dating.**