Unstable mutation occurs when a mutant changes back, or continues changing, without being exposed to mutagen.
Leonardo during unstable mutation.
In the 1987-1996 series, Splinter experiences unstable mutation at the end of Splinter No More, becoming human once more. April O'Neil's temporary mutation into cat in The Catwoman from Channel 6 causes her to become human once again at the end. When the Rat King turns Irma and Vernon into Were-Rats, the mutation is also unstable.
During seasons 9-10, the turtles undergo unstable mutations, becoming monster-like versions of themselves.