Online play can bring up people's inner asshole I presume, since the most enfuriating moment in my gaming career was in WoW.
I bit of background information for the folks around that haven't played it: on your average battleground (15 versus 15, keep control of up to 4 towers in this case) you'll have those people (reffered to as 'leechers') that are too damn lazy to do anything so they just walk around a bit and do bugger-all to help and win.
Me playing a healer in relatively newbish gear meant that whenever I was left with even a single Horde, I was history. About five minutes in the game I notice a certain rogue hasn't been doing all that much outside of intensely staring at a wall, so I look up the scoreboard and see he has indeed done zero damage. Since rogues can't do anything else then deal damage, I asked him what he was doing. He claimed openly to be leeching, since he apparently wasn't useful in any way without having leeched together a set of good gear. Now this is already quite annoying, since whenever enough people reported him for doing nothing, he ran up to the nearest horde and tossed a knife up it's backside, then ran back and proceeded to do nothing.
Where it got infuriating beyond any limits is when aforementioned rogue started complaining that I wasn't useful at all. No matter what I said, he was solidly convinced that him leeching and doing bugger-all was somehow more productive then me grouping up with some others and (attempting to) keep them alive. He kept nagging about my low amount of stamina and the fact that it rendered me useless, and he was more useful because he could sap things (incapacitate them for 10 seconds, any damage breaking it). When I did ask him why he then didn't sap he replied 'because I'm leeching'
a leecher complaining about someone not being useful, oh sweet irony...
