Garen's E
This might not really be a groundbreaking bug, but I felt the need to report it as it's not supposed to be like this ( I think... )
When you use Garen's E right before the game ends ( Victory/defeat shows up etc. ) You'll still inflict damage on whatever champion in your range, even though the game has already ended.
This could ( and yes, it can, it happend to me yesterday ) result in your still killing a champion a few seconds after the victory/defeat has came up.