Static Meshes that are reimported with less materials can lead to map check errors on instances that had overriden materials that require manual cleanup

StaticMeshActors placed in the world can override the default materials of a static mesh. If the static mesh is then reimported with less materials, the actors maintain references to unused material overrides. These references can cause otherwise unused materials to be included in a cook.

The static mesh import dialogue does not offer a way to remap or otherwise remove these unwanted material overrides and resaving WorldPartition actors does not remedy the issue either.

Create a static mesh with more than one material slot.
Place an instance of this mesh in the world.
Override all materials.
Reimport the mesh with less materials.
Run MapCheck and observe the warnings.