Sega Master System / Mark III / Game Gear
This game (the only one on the SMS/GG?) requires that bit 3 & 5 (undocumented X & Y flag) in the F register are preserved when pushing and popping, otherwise it will crash upon game start. It does not rely on correct emulation of these flags.
The assembly code seems to have been compiled from a higher-level language. There is heavy use of the stack for local variables and code sequences that a human would (hopefully) never write, such as lots of sign handling code where it's not needed.
There are some remnants of some cheat passwords. At offset $7dff we see the the words
and the password code checks to see if the entered password matches these (
call $762e at $76e9; returns c=1 for a match). However, the code then seems to nullify the effect. If you patch $76f0=0, entering any of these passwords shows the game ending. It is unclear if there is some way to get different effects for the different words.