The most famous example of this is the minus world, but I find most older games have this glitch. Where the hex byte(s) support warping to more levels that exist in the game, you can warp to glitchy levels.
And there’s also those games where if you are able to go out of bounds enough, you can be met with a scrambled mess of blocks and enemies. Example: https://i.imgur.com/rjiKEmv.png
But why does all this happen? I know why you can access them, but what code in these games loads all of these glitchy levels and blocks rather than it going to a black screen?