So maybe you got in this situation once, maybe you punched the console out of frustration or something else and was greeted with a level select screen. For a lot of time, we thought that this is some hidden hack but the explanation is far cooler.
When developing games on the Genesis, Sega required intense testing for bugs and freezes and would send games back if they found any. That would result in delays and this programmer of some games (including Sonic 3D Blast) programmed the games to send unresolvable errors to “secret stages”. In this case, a level select menu. Moving or hitting the cartridge would break the connection, that would cause an error and the level select is actually an error screen. Here is a video explaining the whole thing.