I wasn't accepted very well by my 'now' ex's friends. Couldn't put my finger on it at the time as I was very polite, I bought a round when I first met them and listened to them, showed interest in the boring cr*p that some of them were talking about and was able to show this by asking relevant questions.
Basically, I did the text-book stuff that you have to do to get on with your partner's mates.
Turns out they didn't like me because I hadn't grown up them, or hadn't gone to uni with them, or hadn't smoked joints with them in college (yes, this was one of the reasons).
Caused a hooooge argument when I said to my girl that I had not desire to meet that bunch of selfish, stuck up, stoned a*seholes ever again. It instantly cut the time in half that I could see her and was one of the reasons cited when we broke up.
It sucks how her / his mates can affect things.
Oh, have since found out that half of them are now divorced, one is in prison and another 'came out' to his girlfriend two days after I'd met him. Hope it wasn't because of me. :)