I got my eyes opened on this last year. I was part of a pretty tight-knit group of friends. Some of us decided to get together and apply to YC. It's a good thing that didn't go very far, because a few months later, one of our group got in pretty serious trouble with the law. I took the view that he was a good person that didn't deserve to have a bad decision follow him around the rest of his life; my girlfriend and I attended his court dates and argued for his parole, and when he was released, we moved him to our area and helped him get back on his feet.
Our previously close-knit group of friends, it turns out, were really unhappy about that. They quit talking to us altogether, even though, as climbing partners, we've been responsible for each-others' lives on several occasions, and have generally been there for each-other through good times and bad.
Never saw that coming. Suddenly partnerships looked a lot less wise afterward.