I was playing as Teemo last game, owning Karthus in mid, and 3 times when i killed him, it said he killed himself, he got credit, i didnt, major bug right thar

dont ignite him. for some reason it hasnt been fixed yet

They know it.
Just learn to look the first page of the forum ?

BIG TIP:
This bug is predictable 100% of the time: If karthus dies to a poison damage source, EI teemo shroom ticks, poison, ignite, mord ult, vlad dot etc., he gets the credit for his own death (and recieves killing spree bonus gold). If however, he dies to a direct damage source, the kill is credited to the source of the said damage.