I was playing my dinomancer deck
new dino
Class: Warlock
Format: Wild
2x (0) Raise Dead
2x (1) Kobold Librarian
2x (1) Mortal Coil
2x (2) Defile
2x (2) Expired Merchant
2x (3) Backfire
2x (3) Dark Skies
2x (3) Free Admission
2x (3) Luckysoul Hoarder
1x (3) Tamsin Roame
2x (4) Cascading Disaster
2x (4) Hysteria
2x (4) Lesser Amethyst Spellstone
1x (4) Spiritsinger Umbra
2x (4) The Dark Portal
2x (6) Cruel Dinomancer
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My opponent was playing quest murlocs.
A random Mutanus appeared in my hand on turn 7 after my opponent completed his quest but before he played the reward.
When my opponent played megafin, I cast hysteria on it and my dinomancer then randomly came back as mutanus after dying (but I never played the card or discarded it) and added a second Mutanus to my hand.
The next dynomancer I played met a similar fate after dying to toxin flurgl combo, it also came back as mutanus and added a third copy to my hand. I have a link to the hsreplay replay if you want it, I cant include it here.