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The death overlay is applied after Kog'Maw dies, after his passive completes. This happened in a bot game and only occured when the Kog'Maw died, not on any other champion.
The problem persisted until your champion died and respawned.
The problem was reapplied when Kog'maw next died.
I do not know if this problem exists with all Kog' Maws or is limited to Bots.
Step's to reproduce:
Play with Kog' Maw bot.
Kill Kog' Maw bot.
This bug is known, it also is applied with Karthus and Zyra, last I checked (anyone with a passive that activates as/after they die?) and I thought there was a thread already on it, but after going to page 4 or so, I couldn't find it! Perhaps I'm blind, or perhaps you were the first one to actually post it!