While playing one of the dark world stages in Level 1 of Super Meat Boy, I reached the end of the stage after dying several times. However, Band-Aid Girl, instead of doing her normal crying animation, was messed up in appearance: glitchy and spinning around. Though I initially hesitated, I brought myself to touch her and complete the level, only to find myself in World -1. This stage was quite difficult and featured arcade-style play with only three lives as with the world zones. When I got game over and went back to the map screen, this stage was now accessible as a stage before the first stage of the Forest light world stages.
I managed to unlock the same sort of stage in the Salt Factory world.
I'm guessing that there are six of these, one for each of the normal worlds as well as the secret world. But I'm still not sure as to how I go about unlocking them.
Under what circumstances does Band-Aid Girl become glitched up, allowing me to unlock the negative levels? Is it determined by chance or do I have to die/not die a certain number of times to get her to become glitchy? Does this only occur on certain levels?