In the 1987-1996 series, Splinter experiences unstable mutation in Splinter No More, reverting back to his state as a mutant rat. April O'Neil 's temporary mutation into a cat in The Catwoman from Channel 6 causes her to become human once again at the end. When the Rat King turned Irma and Vernon into were-rats in Were-Rats from Channel 6, their mutations were also unstable and appeared to be temporary.
When the Shredder first mutates himself, his mutant form was unstable during the events of the 2012 TV series episodes, "The Super Shredder" and "Darkest Plight". His mutation was stabilized in "Requiem".