Textures are Missing

Some unit and tile textures appear with no detail, only a black silhoutte. The game performance may also appear slower than on previous plays, periodically freeze, or crash.

Workaround: 

Clear the game cache files.  You can do this by starting the game and immediately holding down the shift key until the logo screens have appeared.  This will remove any potentially corrupted cached files and allow the data to be reloaded.