Cuckoo (slang)

Something that was where it shouldn't be was sometimes referred to as a "cuckoo in the nest", or simply a "cuckoo".

The Seventh Doctor once called an extra-terrestrial fossil in the caves around the Lifton coast a "cuckoo [that needed to be pushed] out of the nest of human history", else Charles Darwin's The Origin of the Species might never be written, since the fossil could not be integrated into Darwin's theory of evolution. (COMIC: Cuckoo)

Augustus Hayes used the phrase "cuckoo in the nest" in a note to Alistair Gordon Lethbridge-Stewart. (PROSE: The Devil Goblins from Neptune)