Different from the regular cheatcodes you find DEADC0DES for the Action Replay beginning from MK II. They look very unusual, but a quick google research shows: Nothing. The only thing you find about it in the Action Replay manuals, besides the deadcodes, is the following text:
It is important to note that the codes should be in the correct order and no spaces between them. Deadcodes are the only situation where this is important. It is important that you enter the first line ‘DEADC0DE' as well as the rest. You can enter only one deadcode per game.
Another hint gives us the packaging of MK III:
NEW - 'DEAD CODE' GENERATOR: "Dead Codes" are super cheats which effictively enables you to re-write whole sections of the game giving total control. No other product features "Dead Codes".
Sounds good, right? But where is the generator? How to write dead codes? Nobody knows.