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I came across this before I started learning 6502... maybe you've seen it already?
It's a trick where by manipulating the bytes in memory, and then exploiting a bug, the player can cause the manipulated bytes to be executed!
The video goes into a lot of detail of the 6502 commands executed, and also discusses the way the game engine handles sprites and enemy movement in memory...
I also think it's a great example of how random the cause of crashes can be, and how you may think you've debugged everything, but one little bug may still be sneaking around!
What a thing to find; I imagine it was found with a disassembler and some patience, whilst generally looking for stuff, rather than because anyone stumbled over weird behaviour and decided to go spelunking to see what was happening.
robsoft : weareroad.co.uk
That's quite interesting. It really highlights what happens when the program counter goes somewhere it shouldn't. (The program counter will read anything it sees as executable code. Even if it's the game's graphics, zero page ram, etc. This often happens if you RTS without a JSR first)