Some desultory googling turned up: * https://www.nesdev.org/wiki/CPU_unofficial_opcodes#Games_using_unofficial_opcodes https://www.nesdev.org/wiki/CPU_unoffici

Some desultory googling turned up: * https://www.nesdev.org/wiki/CPU_unofficial_opcodes#Games_using_unofficial_opcodes https://www.nesdev.org/wiki/CPU_unofficial_opcodes#Games_usi... * https://hitmen.c02.at/files/docs/c64/NoMoreSecrets-NMOS6510UnintendedOpcodes-20162412.pdf https://hitmen.c02.at/files/docs/c64/NoMoreSecrets-NMOS6510U... (doesn't name any software, but some copy protection schemes were already known to use them)