Why does punching Sonic 3D trigger a Secret Level Select? (youtube.com)

Ah, how I hate videos. This could've been explained so much better using a written blog post...

He basically tricked the SEGA QA process...

Summary: Sega QA tested games and rejected any game that crashed, so the developer 'try/catch'ed* all errors into the level select.

Punching or wiggling the cartridge triggers one such error state.

* Pointed all error interrupts to the level select.

You can get to the level select screen in Sonic 3 using a similar process.

1. Open Sonic 2 and enter the level select cheat

2. Freeze the game by wiggling the cartridge

3. Replace the cartridge with Sonic 3, and hit the reset button

This blew my mind as a kid.