I've discovered the issue behind Kat Q getting bugged to where you can't use it.

It's very hard to replicate but it can be done and I have done it now.

Step 1 - Pick kat and be past level 6 with ult not on cooldown. (I've only had it happen with ultimate up) Step 2 - Have an Ally help because it only happens when an ally takes the kill. (possibly because the only way i could take it while q is in animation would be ignite) Step 3 - With low hp enemy press Q. Step 4 - Have the ally take the kill at the very last second of the Q animation as the blade is in the air before Q is actually CD.

The bug has to do with Kat's passive resetting Q when it's not actually on Cooldown but because of how Kat's Q animation vs CD works it bugs out in a small window of time where it gets reset when it's not actually on CD.