The Great Giana Sisters:

The Great Giana Sisters was never officially released for the ZX Spectrum, 
despite a port being developed and reviewed by magazines in 1988. 

It was created by Ian Richards of Source Software for UK publisher Go! (UK) 
and Rainbow Arts, but the game was pulled from shelves shortly after review 
copies were distributed, reportedly due to a warning letter from Nintendo 
regarding its similarities to Super Mario Bros.

While magazines like CRASH (92%) and Your Sinclair (8/10) reviewed the 
unreleased Spectrum version, no commercial copies were ever sold to the 
public, making it a legendary unreleased title for the platform.