Pin #14 is the CPU R/W pin, and if it's not properly connected, the game will be unable to write to the MMC1 mapper to perform bank switching. Platoon happens to be programmed in a way that address 0x8000 of every bank is an entry point that will run a particular level from the game. So you boot up the game, and it tries to switch to the Title Screen bank, then jumps to 0x8000. But the bank switch fails, and instead it runs code from the first bank. It just so happens that the first bank contains the program for the ending.
If the cartridge connection improves and mapper writes start to succeed, the graphics will return to normal as it continues to run the ending.
How would reading a scratched/dirty disc permanently damage a console? That seems like a very bad issue for a device expected to read frequently swapped discs.
Him: So, have you ever thought about basing the hacking mechanics on Hyrum's Law?
Me: ...No, but I'm sure that if it ever develops a speedrunning community, they will do just that!