Celdor

Celdor was a planet that the Poldagon scanned and recreated on their research planet. An incarnation of the Doctor visited the Poldagon planet while it was in the form of Celdor and believed himself to be on Celdor. When the Seventh Doctor tried to visit the planet again, he returned to the Poldagon planet and learned that he's never been to Celdor. (AUDIO: The Doomsday Quatrain)