The term Failed Earth refers to a planet or moon that failed its terraforming process, for a reason or another. There are many reason why this might happen. The term is not used in science, it is more often used by terraformers.
Snowball Earth Edit
Main article: Snowball Earth.
A runaway ice age can, mainly in case of an Outer Planet, end-up with temperatures that make life on the surface impossible.
Fireball Earth Edit
Main article: Fireball Earth.
A runaway greenhouse effect can turn a planet into a new Venus.
Short Lived Earth Edit
Main article: Short Lived Earth
A terraformed planet that is not maintained properly or has some major problems that in the end will disrupt its equilibrium up to a limit where nothing can be done to reverse it.
Runaway Earth Edit
A planet can be diverted from its initial orbit and sent to forever wonder through the galaxy. Also, a Rough planet can be formed in the interstellar environment and at some point might get close enough to a star for a limited time, to sustain terraforming.