Minecraft wiki entry for Placeholder_texture, associated text: In some cases where textures were not explicitly defined for certain blocks, the blocks in question would "default" to the texture at position (0,0) of the texture atlas in use. Where terrain.png is used, this was the top of the grass block, however with the deprecation of predefined texture atlases and their replacement with atlases which are generated at runtime, this texture tended to shift with time.