The Turtles' unstable mutation forms, debuting in the episode "The New Mutation".
Unstable mutation is a mutation caused by a mutagen source that has unstable properties. It is a concept that was introduced as an ongoing plotline in Season 9-10 of the Red Sky seasons.
It revolves around the the Ninja Turtles undergoing secondary mutations due to the mutagen that they originally mutate from has become unstable. This leads to them mutating into more monstrous and reptilian versions of themselves, temporarily giving them brute strength and unique abilities for a short amount of time. It was only later during "Doomquest" when these mutations were causing the turtles to become more uncontrollable when these unstable mutations were diminishing the Turtle's intelligence while in this state. This led to the Turtles using the Vortex Crystal to stabilize their mutations and prevent them from mutating into these forms again.
In the 2012 series, Super Shredder also dealt with his own unstable mutation due to using super mutagen that was yet to be perfected. This caused his left arm to become deformed and he had to take more mutagen to remain stable until the dosage could be perfected.