This happens sometimes, especially when working with large assemblies.  I hide a bunch of parts and then they are still visible on screen.  In the tree they are non-colored which is what you would expect for a hidden part, so I know that solidworks thinks they are hidden.  They disappear while panning or rotating with the 3rd mouse button, but then pop visible again when I let go.  My workaround has been to close and reopen and that sometimes fixes it.  Is this a graphics card problem or what's going on here?