QWER won't cast
Usually it happens as soon as the match starts. Any spell I attempt to cast simply doesn't work - the only way I can get it to cast is to click on the spell symbol on screen, then point it where I want it to go. Sometimes this bug will happen with specific abilities later on in the match after I respawn (so far I've only experienced that with Xerath's Q and the reworked Sion's Q). It's a very annoying bug, and almost always causes me to lose lane or the game.
The only fix I've found for this bug is to completely close the client, relog, and reconnect.