How does `error.mdl` relate to other common Source Engine error visuals like purple/black checkerboards?

`error.mdl` signifies a missing *model*. The purple/black checkerboard pattern, on the other hand, typically indicates a missing or corrupted *texture or material*. If `error.mdl` itself appears with a purple/black checkerboard pattern, it's a meta-error, meaning the model is being displayed because another asset is missing, but even the textures for `error.mdl` itself (usually `error.vtf` and `error.vmt`) are also missing or corrupted.