Etrata the Silencer trigger + Mutate.
This is an old bug, I need to reproduce it again and will, but I don't think anyone else has ever discussed it or if it has been addressed. To reproduce: Attack an opponent with Etrata. Hold priority while her trigger is on the stack. Flash a mutate creature on Etrata and choose TOP.

EXPECTED OUTCOME: Etrata and the mutate card both shuffle into the library. Target is exiled with a hit counter.
ACTUAL OUTCOME: Target is still exiled, but the Etrata mutate stack stays on the battlefield. This appears to be because it is a new creature and the interface appears to be treating it as a new game object.

NOTE: Flashing a mutate creature to BOTTOM of the Etrata pile does not work, both cards do shuffle in.

I will play a few more games and attempt to reproduce again.