Ohoho, I guess know what's up.
It's the good old dreaded Megadrive red screen of death, isn't it?
That's Sega's anti-piracy scheme.
Megadrive roms have an internal checksum calculated from rom data, and naturally tampering with rom data messes it up. There are tools
to fix that checksum whenever modders are done editing their roms.
Besides a wrong checksum, the red screen of death can also be triggered with cheat codes enabled at startup in some cases, with some emulators.