Minecraft wiki entry for Stitched_items.png, associated text: Whether it was actually used for item textures is unknown, as it can be removed from .minecraft at runtime seemingly without affecting the game. In addition it 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_items.png in 13w09a further supports this theory.