All too often....
I tend to step in when I see some un-necessary aggression going on between 2 people, especially if it looks like it's one person being attacked, rather than just 2 dickheads going at each other.
In the case of one person being antagonised by another, I would normally wander up and ask the "victim" if they are OK, often this gets an obnoxious response from the aggressor, which I'll generally ignore or pointedly tell them that I wasn't asking them, and then ask the question again. If the response is that they are OK, then fine, I'll walk away, otherwise I'll do what I can to make sure it doesn't turn ugly.
I have in the past, found myself stepping in and pulling two people apart who are already in the throws of punching the shit out of each other (Two Tribes earlier this year was a good example), and once they calm down (or if I just get told by all parties to fuck off and leave them alone), then I'll go.
Whenever I do step in, it's generally in a polite but firm, and non-aggressive manner. Given how often I've found myself doing it (most of the time without really even thinking about it) I've sometimes thought that it's pretty amazing I've never had the crap kicked out of me for interfering.
Having said all of that, I've never been in a fight, and have never punched someone out of anger/aggression. The most I have got is a half arsed punch to the face or similar, which is normally followed by the puncher fucking off immediately afterwards anyway.
