Was at a game a few years ago where about 5 blokes, proper hard cases, probably drunk, were shouting obscenities at the opposition's goal keeper (really nasty stuff) some guy there with his 2 kids asked them (politely) to stop because of his kids and they started to lay into him and the one security guy in that section was obviously too afraid to get involved and just radioing for help which wasn't coming. Got to the point where we were like we are going to have to do something then finally the police turned up. (EDIT: Sadly in general something which seems to happen far too often and one of the reasons I don't tend to go to games much - not out of concerns for my safety just I don't need to be around that or take other people to be around that).
Another time had a bunch of lads jumping and bashing stuff, there was quite a group of them and really going to town, making as much noise and chaos as they could resulting in the whole stands shaking and I was concerned as to how well they'd hold up, sure in theory they should but it wouldn't be the first time they've not - they were too stupid to have any such concerns.