Well, I hated one of my best friends for something she did for more then a year. However, we both got over it and are now closer then ever.
When we were'nt friends though, I seriously hated her, loathed her even. I remember I was in a pub once with some friends, (it was my birthday, we were about to have a big night out) and she walked in aswell (our B-day's are very close together), and I just got up and walked straight out and went home. She ruined my whole night!

But we were never exactly enemies. We never had a huge bitch fight, In fact, when we orginally fell out, I informed her our friendship was over in a pretty damn polite email!

I've permamently fallen out with ppl before, but no-one's ever become what I'd call an 'enemy'.