Placeholder texture

Minecraft uses an assortment of different placeholder textures for handling cases where textures cannot be drawn normally.

Missing texture
Perhaps the most famous of all cases, the missing texture, a texture composed of a black and magenta checkerboard, turns up anytime a texture or model reference could not be correctly resolved,

Animation placeholder textures
Another type of placeholder texture, these were overwritten by the actual animated textures during gameplay once said textures were generated by the game. Needless to say, they would appear themselves if the animation could not be generated for whatever reason.

Texture atlas blank spaces
When predefined texture atlases were used for storing and loading textures, certain patterns were used for marking unused regions on some atlases, where others simply used outright blank spaces.

These texture spots end up being removed as the texture atlases containing them are split up into individual files.

Default texture
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.

Since 14w25a, all such cases are instead directed to the missing texture.

Uses
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Bedrock Edition missing textures
Unlike in Java Edition, which only has a single missing texture which is generated by the game at runtime, Bedrock Edition defines several textures as image files in the game's assets directory.