The Simpsons: Bart vs. the Space Mutants (lost prototype of Game Gear port of "The Simpsons" game; 1991-1992)
The Simpsons: Bart vs. the Space Mutants is a platform video game developed by Acclaim and Flying Edge released in 1991 and 1992 for the ZX Spectrum, Amstrad CPC, Commodore 64, NES, Game Gear, Master System, Atari ST, Amiga, Genesis/Mega Drive, and the PC. The game has received mixed reviews.
However, screenshots of a completely different version (possibly an early version) of the Game Gear release (which is similar to the Sega Genesis version) is featured in the manual of the US Game Gear release of Bart vs. the Space Mutants. This version is quite similar to the NES and computer versions, especially with a similar title screen.
Given the fact that the popularity of classic Simpsons episodes and media are declining, it is very unlikely that the prototype Game Gear version will ever be found and/or released. Black and white screenshots in the manual and promotion magazines are the only evidence of the prototype’s existence.