Undeserved taunts. It really shouldn't make me that salty but man, it gets under my skin.
I played against a Dedede online that I slipped up and he killed me with a f-smash when I had a full stock lead and he was at like 90%. He taunted. Needless to say I went absolutely ham on him rest of the match and beat him easily. The next round he went Diddy out of pure sodium chloride, and I switched to D3 to show him what a real D3 main looks like. Two stocked him, then went Diddy and 12% two stocked his garbage Diddy, taunting after both "kills" (I think he SDed twice). Wrecked his Lucina with Diddy again, then he left.
Yeah, don't taunt against me unless you took the lead on a nice kill or won the match. I realize my actions were just as childish as his, but I needed to show him he had no reason to be taunting me.
Another is when you lose to someone good and you want to keep playing their main, but they switch to your character to prove you have no idea to play the character (ironic following my last paragraph, I know. I only do it if you piss me off and happen to be playing a character I know). I had someone Greninja ditto me once after beating me with Ness, and I 4% two-stocked him. Don't do that.