Xerath W/Q Bug
I've been playing Xerath lately, so for now I'm going to assume this has been since patch 5.7. I've been noticing with Xerath, sometimes if you begin casting your Q immediately after casting your W, your Q will cancel itself and not go on cooldown. It seems to fully refund the mana as well. The animation for the Q and the self applied CC still occurs, but just cancels.