Just made this Game Genie code, so have only tested it for the first 60 seconds of the game, but seems to work fine.
Sonic the hedgehog (Genesis)
Die in one hit (even if you have rings).
This is what the Game Genie code is doing...
01:AFFA 4A 78 TST.W ($FE20)
01:AFFE 67 00 BEQ #$0090 [01:B090]
That first instruction is testing the RAM address for rings (FFFE20). The second instruction is telling the game to branch 90 bytes if the ring count is equal to zero. This will branch to the death routine. If it's not equal to zero, it doesn't branch. The Game Genie code tells it to always branch no matter what (as if your ring count was zero).