A glitch level (or glitch zone, or negative zone) is a hidden bonus stage in any chapter (other than The Cotton Alley) which is like a warp zone but generally more difficult to complete, and harder to unlock.
The four warp zones in each chapter must be found within standard levels of that chapter in order to be unlocked, and they are always found in the same places within the four levels. By contrast, a glitch level for a chapter (there is only one glitch level per chapter) only becomes available to unlock once the boss fight for that chapter has been completed. Upon completion of the boss fight, replaying any level of that chapter gives a small chance that Bandage Girl will be "glitched out", with distorted appearance and making strange beeping noises. If she is in a glitched out state, the player must reach her without dying, for dying will reset the level, and she will likely not be glitched in the reset level. Reaching her in her glitched out state will take the player to the glitch level.
Every time the player dies and respawns in a level, Bandage Girl and the entire level are reset, and with each reset she has an equal (but low) chance to be glitched, so a convenient way to unlock glitch levels after a boss fight is to pick an easy-ish level within that chapter in which Meat Boy spawns in a position where Bandage Girl is already visible. Then, for every time she is not visibly glitched, deliberately kill yourself as quickly as possible. Doing this repeatedly will eventually give you a glitched Bandage Girl, and since she is visible from the start of the level, one is saved from rerunning the entire level a large number of times only for Bandage Girl to be glitched in a small number of those attempts.
Glitch levels work much like warp zones, giving you three attempts to run through an 8-bit style level. What makes them exceptionally difficult is that while warp zones are broken into three stages, each of which gives you three attempts, glitch zones have only one extended stage approximately the size of three warp zone stages end-to-end, and you only have three attempts to complete the entire glitch zone.
All text when playing a glitch level is very scrambled.
Bug: If you get a game over, then mash a, when you splat, you will enter the glitch level, but the map music will keep playing until the BSoD shows up.
Another method of reaching a glitch level is by "playing" a map's starting platform after the boss for that map has been defeated.
Translations: Sorry Meat Boy, But Bandage Girl Is In Another Warp Zone.