I was on level 5, fighting Masofuze, and used my Aflatoxin card to take out his Graviton Fire-Eater.
But the resulting Malignant Cell refused to die. Twice. When hit with lethal damage (igniting Unstable Gas, both times, courtesy of a Blue Nymph), the expected -1 and death animations happened, but a fresh new Malignant Cell appeared in its' place. As far as I can tell, the Aflatoxin effect that created it remained in the space without any icon.
The other, younger Cells that it spawned died normally to the Gas.