Over the last month I've been facing a bug with Katarina's R not working in group fights.
I'd E, W, then R and she will just stand there without using her ult. It's cost me more than a few teamfights as I'd stand there for a second waiting and then when I look at it, nothing. She just stands there like no input went in.
At first I thought it was my ping, but it isn't. EuW isn't the only server I face this on.