Kat BUG
 

She can't Q,R nor B
If she dies, she's able to cast these again, but if she R's then she can't cast them

I can vouch for this. It happened after I got a kill. The enemy Katarina had no kills and could still use Q and R, however, as soon as I got a kill, I was unable to do anything of the kind.

Perhaps it's to do with her passive?

Look at the 6 other threads that have already reported this? ._.

When I looked at the front page I didn't see anything. Whatever though, same thing.
And you should focus on the problem and not on how many times it's been posted

It's not like us commoners can do anything other than inform the gods of Riot. :/