The enemy AI used Drain Life on my Fallen Druid, leaving it with 3/1. I saw this as the AI pushing my hand into Improved Mutating my Fallen Druid, so I did.
The result was a 12/6 Crimson Dragon with a Deja Vu ability. I then activated Deja Vu to get the 12/6 Crimson Dragon with the Burrow ability. To top it all off, I used Parallel Universe on the Burrow Crimson Dragon in order to create the 16/8 Crimson Dragon with Mitosis ability.
Unfortunately, the AI ran out of health before I could activate Mitosis.
The 10/10 Golden Nymph with the Destroy ability was a result of mutating my 1/1 Forest Spirit.
Edit: As a side note, I've actually come across a bizarre mutation. It was a 4/5 Shrieker with the Guard ability. Instead of a normal Guard, though, the Shrieker would instantly kill anything it used the Guard ability on. The Guard animation was there, but the creatures would just die instantly.