The secret at the first level (that leads the player to the Black Market) used to skip almost entirely the whole game. After the Black Market, the player would be led to level 7. 

I eventually noticed that by being a hard game, most players wouldn't give it a second chance, and if they did use the skip, most of the game's content wouldn't be experienced. Content of which, in my opinion, teaches and sharpens the player ability, making the later levels easier.