I just had a user contact me with an error when checking in an assembly.  A sub assembly is marked as "local file is modified" but he hasn't checked out the sub assembly or directly made any changes to the file.  I am not sure how this is happening, and can't recreate it myself.  Has anyone encountered this before, and know what could be causing it?

Image of the properties card below (note the "The file is not checked out." and "Local file modified").