Jayce Mercury Hammer Bug
I'm surprised and annoyed that this hasn't been addressed since 6.4, and it's very likely this has been around for a little while before it, but Jayce's Ultimate Ability Transform: Mercury Hammer creates a staggering lag spike whenever it's performed. And, someone actively used Jayce and this bug as an aggravated response to not getting the champion they wanted to play. I'd like for Jayce to be remedied of this game-ruining bug, as well as disabled until a fix can be found.