The only hints of real grief I've seen have been between anglers on busy commercials. They've usually revolved around a distinct lack of etiquette. Casting into swims other than their own, noise, excessive swearing etc etc.
Other than that, I've had the odd encounter with lippy kids, if I've been unwise enough to fish a stretch of river, a bit too near to town, on a nice, sunny, summer's day.
I've certainly heard of some rather nasty encounters locally, thefts, assaults etc etc. However, I avoid all of the places that such things have happened or where I think they are likely to happen. There are a couple of decent syndicates locally that I wouldn't touch with a bargepole. Both are tempting, because they are both pretty good. I could have got in on either before any incidents occurred, but I shied away. I knew that both had the potential for unsavoury incidents. Sadly, I've been proved right.