It’s pretty interesting how the game can continue to function in a level that isn’t supposed to exist. Metroid has similar areas I believe. It’s hard to imagine a modern game or any modern software handling a scenario like that without crashing.
Just yesterday I was storing my color planes in three separate adjacent buffers for reasons, and if you looped the player off the bottom of the screen it came back up in a different color on the top. Now I need to figure out what to do with that…
The point is this stuff arises whenever you have, like `char level_data[8][4][10][100]` rather than, like `class level { std::vector<actor*> actors; std::vector<sprite*> background; }`. And storing stuff in large contiguous blocks that can be interesting when misinterpreted is still a pretty good way to write modern games if you don't have OO brain rot.
Pretty much the only ways an NES game can "crash" is by calling an invalid function pointer, corrupting the stack, or getting stuck in an infinite loop somewhere. Otherwise, the game will more-or-less stumble on no matter what you throw at it -- accessing an array out of bounds will usually give you a nonsensical value taken from whatever comes after the array on the cartridge, dereferencing an invalid pointer will just give you some arbitrary data from RAM or ROM, etc. Therefore, the effects of going to an out-of-bounds level can vary from game to game, depending on what sort of format the game uses to store level data. Off the top of my head:
- Super Mario Bros. gives you slightly broken versions of regular levels, such as the various Minus Worlds. I haven't looked into the code, but presumably this is because a "level" is really just a header that contains indexes into tables of rooms & enemy lists & whatnot. So, going into an invalid level gives you a weird combination of existing rooms and enemies.
- Super Mario Bros. 3's level format has a lot of function pointers, so it usually crashes if you select an invalid level from the map screen. But the levels get decompressed into a big 2D array of tiles in RAM, so if you go out-of-bounds within a level you end up in a world of garbled tiles. Although some of those tiles will call invalid function pointers & crash the game when you interact with them; famously, this can be turned into an arbitrary code execution exploit. Here's a video of a speedrunner exploiting this in real time, manipulating enemies so that their positions in RAM form a short program that triggers the end cutscene: https://youtu.be/mUeozQtSmrg?t=202
- Mega Man 2 stores level data as a grid of tiles on the cartridge, but all the levels are stored consecutively, so if you go out-of-bounds you usually end up in a garbled version of another level.
- The famous MissingNo. glitch in Pokemon Red and Blue is caused by use of an uninitialized variable that causes a decompression routine to run on bad input, which then triggers a buffer overflow that stomps on part of your save file: https://youtu.be/ZI50XUeN6QE?t=880