Client Crash - Leaver Penalty
I don't mean during a match... I mean when the client up and crashes in queue or champ select from a temporary, inherent flaw... and then you get to sit out the penalty for 15 minutes. (Which is interesting as I have zero leaves or dodges anywhere... not in almost two years.) One would think since the Error Report contains your user information that this would flag the idea that you didn't merely dodge the queue... especially since the error is client faulted.
This happens about once a week. It gets mildly annoying, and it is an easy fix. It is hard to abuse, since forcing the client to register the fact that it errored out is fairly difficult on your own.