64JPX is a small interface designed to connect modern game pads to the Commodore 64 or Atari system. While some game controllers, notably the Sega Master System and Sega Genesis, are physically compatible with the C64 at the connector formfactor and pin level they are not fully electrically compatible. Depressing a button on a Sega controller connected to the C64 while also depressing a key on the keyboard exposes the C64’s CIA chip to voltages it was not designed to handle making damage or failure likely.
Interesting. I thought that Megadrive controllers were meant to be completely compatible with the C64 and 8-bit Atari machines, but apparently not!