Had a party for my 20th on Saturday. At one point I was talking to some friends, and was looking for a tea-towel because two girls needed to dry their hands. I couldn't find one, and they ended up drying their hands on someones shirt. His shirt was soaking by the end of this, and at the time he had been talking to my new flatmate. They were taking quite a liking to each other, and I apparently picked up on this. Not wanting my friend to get with my flatmate for whatever reason (didn't want to complicate things) I insisted my friend come with me to my room to change his shirt. He was hesitant (for obvious reasons), and I ended up dragging him away to my room. Yeah, I was the biggest cock-block ever. So he took of his shirt to swap with one of mine, and one of the girls in the room saw this. She took an interest to him immediately.
20 minutes later they had sex in my bed.
TL;DR: Dragged my friend away from a girl he was hitting it off with; he ended up sleeping with someone in my bed as a result.
What's the worst backfire you've experienced lately?