Troll Surrenders
I can't tell how many times I have had my team surrender at 20 minutes in a ranked match. If you can't be bothered to play for more than 20 minutes, why even play ranked? It is the most frustrating experience in the game, more annoying than having an afk in my opinion, when you do things right in your lane (take the tower, get great cs, get kills, roam and assist other lanes), only to have your team surrender at 20. Please Riot, put something in place that prevents this type of trolling! Reporting people isn't enough!
and then try to give up 15mins into the game when they have a weak early game for some reason. #1 pet peeve.