I had two friends I had problems with (I haven't kept in contact with either of them.)
Friend number one, well, she was nice. I really liked her, but one thing she did really got on my nerves. I would be telling a story, or hell, just chatting in general, and she would finish my sentence with me. EVERY damn time. And the worst part was, she would always say something that was entirely wrong, so not only could you not hear what I was actually trying to say, but it just made both of us sound stupid. That all ended, however, my fourth year of university. As far as I know, she enrolled in classes, and she just never showed up. Not sure where she is anymore.
Friend number two, I met when I was working at a bakery. She was nice, and everyone was friends with her. Looking back, I'm not entirely sure why. She would complain about damn near everything. She had a good life (not the greatest, but it's not like she was homeless or abused or anything), so there wasn't really any reason to be so damn negative all the time. But the straw that broke the camel's back was when I moved away. I added her as a friend on Facebook, and from then on, the only time I ever heard from her was when she wanted something for farmville, or one of those other junk games you see on that site. I could have lived with that, but holy cow it's like that's all she did all day. I'd log in and see thirty notifications from one person and I'd be like "wtf, mate?"
So yeah, dropped her pretty quickly. Of course, after reading all these stories, it sounds like I got off easy.