My team's Volibear is 4/2 in toplane versus Mundo in a close game - that is, incredibly even. I am 4/2 jungle Jax, botlane is holding their own (despite Ashe crying about not having kills), mid is doing okay. We took first Dragon and I took Herald, but we're behind a few kills.
Volibear dies to a well-executed gank. He's now 4/3, still well in control of his lane.
"OMG I go troll bye"
And he does. After less than 20 mins - not enough to judge which way the game will go (mid-game power spikes yet to be achieved, etc).
We proceed to lose the match due to him being incredibly toxic, feeding and even AFK for a while. While Ashe also continues crying about not having kills, also feeding. The other three of us are helpless to stop the snowball despite trying our best.
I learn something I already knew - muting does not stop a player from feeding or trolling, no matter what all the generic 'advice' is you see on the boards. A perfectly-winnable game lost because of YET ANOTHER troll, who of course will not be banned, ever.
Rinse, repeat. Several times.
Welcome to ELO hell.