People who add you to duo...
And then unadd you the moment you lose one game.
I get that these people (Not all are this bad, I'm sure.) only add you because you did well and they, of course, want to increase their chances of winning by duoing with you, but it's still a new level of low to basically throw you into the garbage as soon as you don't completely carry a game.
[zombie-brand-facepalm]
This has happened to me so many times now and i'm sick and tired of it. Small bout of venting over.