3 long-standing Karthus bugs, never been fixed.

I'll just make it short and sweet, but both bugs involve going into or being in your passive: Death Defied. The other one involves trying to cast Q on Scuttle crab

The first makes it so that occasionally, your Q goes on Cooldown for whatever reason. I'm not sure why this happens, but it does happen occasionally. I think it has something to do when Casting Q as you're about to die? (has been around since at least patch 7.15)

https://www.youtube.com/watch?v=5fXtjZWAWLw&feature=youtu.be

The 2nd one involves being in passive and casting W. To illustrate this, I went into practice tool and used dragon to put myself into passive, and casted W behind the wall. Whilst alive, W will give vision at the poles/ends of the wall, illustrated here:

https://i.gyazo.com/a85903bd0a4efb78cd5e05505a8eccc5.jpg

However, when inside passive, the Wall fails to give vision. I believe this to be a bug.

https://i.gyazo.com/8ef94872a19a9b6455aefe3dc9b8c6f7.jpg

Finally, I'm sure you've seen this before, but occasionally Scuttle Crab will simply Ignore Karthus Q's entirely. I'm not sure why this happens, but I can guarantee that this is irritating as hell.

https://plays.tv/s/LvW8amCfViQX

I hope riot sees this, and can give a response.