Anything that I consider to be winning with the lack of skill. That constitutes using a gltich or a bug, a gun that you can "buy" through donating, exploiting, or using a strategy that really doesn't take any skill but works because the game thinks its funny.
Examples of this would be in any WoW private I play, I get absolutely raped by the other factions Donors, and they always corpse camp. Guys in RTS games that come up with some hair brain strategy to sell all of there buildings except for the building that creates INF units and uses all of his money to spam INF. Using a certain gun that is extremely overpowered, like the PKM in BF2, that thing did the most dmg out of any gun and could fire on a point at any distance if the person weilding it dolphined it up.
If someone comes up to me, and kills me, or beats me without having some towering advantage, I.E. the only reason that he wins is because of what he does, not the advantages he has, then I bow to him for being better than me. The other situation like that is when I'm playing tf2 as a medic and a good scout kills me. The scout is just designed to kill a medic in that situation, so I don't get angry about it.
Other than that I don't think its fair that someone else who isn't better than me should be able to beat me, but as Luck and stupid game mechanics have shown, in competitive play, such is not the case.
The thing that really bugs me is when this kind of situation really starts to impede on the basic element of any game, Fun. Getting killed is fine, I mean you can't expect to play a competitive game like that and not die once in a while, but when it gets to the point where you can't even get out of your spawn, or you can't even be of use because there's one guy in a tank right outside your base, then that's where its really bad. I rage quit when that shit starts to happen cuz it absolutely drives me ape shit.
/End Rant