Minecraft wiki entry for Stitched_terrain.png, associated text: Whether it was actually used for block textures is unknown, as it can be removed from .minecraft at runtime seemingly without affecting the game. In addition, a sister atlas, stitched_items.png, has several invisible pixels shown at full intensity. As such, this atlas may just be a debugging byproduct - more information is needed on this matter. Its renaming to debug.stitched_terrain.png in 13w09a further supports this theory.