I was in the red-colored world and ended up pressing the "A" key by accident, which brought me back a couple of levels. That didn't bother me at all, so I decided to explore what other keys did unexpected things, that led me to press the "1" key, and that brought me to the very beginning of the game.
My question is, why does the "1" key do that, and the other number keys don't take me to previous levels I've finished? Also, If there is a go back/skip level button, I would explain those in a simple README.txt document.
Last thing, the speed-run and practice keys don't seems to work, is that because I must beat the game before using them?
Thanks for your time, and this is a great game!