Check the final level of Fairyland Story ending on MamEnd.
The background graphics are quite obviously broken in the top left corner! Strange that nobody reported this bug.
Anyway, I've fixed it now.
I've also been studying what the MCU does in that game, and the programmers made some interesting choices.
First of all, the MCU can return data stored in its internal ROM. This is done with short tables that are needed at the beginning of a level, and when you die. More interestingly, the text needed for the continue screen is stored there, and since you can't continue until level 8, it would take some time for a bootlegger to notice the problem. Even more interestingly, the endgame text is stored there, so one would have to play all the 101 levels to get to the protection.
Another wicked part of the MCU protection is that before action starts in a level, the game takes three bytes, performs complicated calculations on them to convert them into an 8-byte number passed to the MCU, and the MCU then performs more complicated calculations, recovers the original bytes and sends them back. A lot of work to essentially do nothing.
Finally, during the final level, the MCU is used to calculate and update the trajectory of the dragon's fireballs. Without the MCU, the dragon doesn't fire. I just have to wonder: why bother? How many people would have reached the final level in the arcades? And would bootleggers have bothered anyway? Figure some player going to the arcade owner and asking for her quarter back because the ending was too easy :)