Last March presented me with this one. Someone I had amazingly good reason to passionately despise (something he did ended up cutting me out of £325; shan't bore you with the details) was gonna fail a uni module unless he got someone to stand in for an actor who pulled out at the last minute and screwed everything up. Had this happened he would've had to repeat the year, which would've really made mine because I'm a vengeful bastard. However, a very close friend of mine was working with him. So when asked to help I was faced with two options:

1. Letting someone I really hated fail the year and repeat it

2. Stopping said someone from dragging one of my best friends down with him to share his fate

Went with option 2. I've always been a little bitter about it but letting my friend get dragged down with him would've sucked a lot harder. Besides, having someone you don't like be indebted to you is pretty cool.