This is a weird one, but it's something that happens, so I'm gonna say it:
People who don't use the character they're best with just because there's a better MU.
Seriously, I have never touched DK, decided to pick him up literally (that needs underlining for emphasis - literally) an hour ago, run into a guy who counter-picks Sheik after doing respectably well with every other character he played, except Sheik. -and I mean it, my DK is not good, at all, and this guy was getting bodied (as Sheik) by me as DK, a character I picked up an hour ago...
I'm baffled, I'm genuinely baffled. Why counterpick a character you're awful as just because the game you just lost was literally a smash attack away from being yours?..
-actually, that in general. Why feel the need to counter-pick or re-pick again and again at all if you were doing really well? It's something I will never get. I at least tend to give a character a best out of 3 before I put them down and admit that character is defeated. It's like these people don't see or appreciate a close game when they get one. They obviously just see "all or nothing" which is totally wrong and potentially harmful to your skill. *sigh* Oh, FG people... -you disappoint me.
[Edit]: Oh, and I'm going to mention triple-taunting again because it's [SMAAAASH!!]ing annoying and keeps happening to me. One is enough, chill your ****, you egomaniac.