Happened again against a mono black deck, with much additional strangeness:
Cast 3 mutations on opponents Minor Vampires, everything as normal.
On his turn, he played about 10 Minor Vampires (!). When he hit "DONE", there was the sound of a really loud POP!, and about 4-5 of the Minor Vampires he had just played disappeared, 2 of the 3 creatures I had mutated disappeared, and the one that was remaining mutated into a different (stronger) creature. Also, my opponent ended the turn with -38 black quanta and -12 aether quanta. (Those are negative numbers). Very weird.