Every time Ryze killed me as Karthus it gave him a double kill and registered as 2 deaths for me.

The Ryze had a manamura (and it was on) and his ult was used all 3 times when we tried this.

Only Ryze could do the double kill thing. Not any other champs.

Edit: He had that cleanse item from chalice, tear, ROA, scorcs. That's all i really remember.

Wow, man, that must have been so frustrating. Do you have the other items he had? I wanna try to rep this.

might be the same as the old glitch that happened with kog'maw/vayne and shaco, where kog'maws deaths counted as double and he exploded instantly...

then again maybe not. Might check this tomorrow

I saw this happen with Zyra somewhat recently. Someone killed her and it showed that they got killed twice.

Here's another thread mentioning the same bug... and again with Ryze

http://na.leagueoflegends.com/board/....php?t=3060717

This just happen to a person in one of my games, Ryze killed Kog'Maw and when his passive blew up it gave Ryze a double kill.

Had a akali in my 1 of my games , akali killed a karthus an it said double kill.

Last time i saw this bug was with kogmaw/karthus

I just managed to reproduce this succesfully but only once did I manage.
I had a muramana turned on and had ult activated when the killing blow happened, how ever I didn't see what dealt the killing blow. After I had done it successfully I tried just killing with the Q which did not do it again. I think it might be the muramana autoattack but not entirely sure.