Why These Bugs Exist

Every glitch in this museum traces back to two decisions that made total sense for a 1985 cartridge with a few kilobytes to work with, and just happen to leave gaps a determined player can find.

Collision is checked once per frame, not continuously

The game doesn't ask "is Mario about to hit something?" — it asks, roughly 60 times a second, "is Mario currently overlapping something?" Move him far enough in a single frame (which underwater physics, with its slower and floatier motion, makes easier to land precisely) and it's possible to briefly occupy space the level designer never expected anyone to stand in before the very next frame corrects it. That single frame is the entire gap the wall clips live in.

The world number is just a byte, and bytes don't validate themselves

The level-select routine reads "which world am I in" from one number in memory and trusts it. Under normal play, that number is always something the game itself wrote there, so it's always valid. The Minus World is what happens when a wall clip lets the game load a pipe destination that was never meant to be assigned a real value — the routine doesn't know the number is bad, so it draws whatever's there using the tile reserved for a coin-counter digit, which is where the "−1" comes from.

The takeaway

None of this needed a "bug" in the sense of broken code — the collision and level-select systems both do exactly what they were written to do. The glitches show up in the space between "what the code checks" and "what the level designer assumed a player would never attempt."