Pissed people who don't know when to shut it.
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Pissed people throwing beer over people who dont want it/need it/ask for it/deserve it.
...and the threats and general nastiness that go with it when you object to it... (PSC match) 
Instant fight.
Pissed idiot spills beer on you.
You turn around and go "Oi, f**kin' watch it, dickhead!"
They come back with "f**kin' calm down, it was an accident."
You turn around and they do it again 'for a laugh'.
And then it's on.
This scenario must get played out at every sporting event in the world where there is a) Men and b) Beer.