Have you ever gotten a file from someone else and seen a "Drawing file is not valid" warning message when you try to open it?

This occurs when you are using the 2006 or earlier versions of AutoCAD to open a 2007 file format drawing.  Unfortunately, the only way to correct this is to have the file saved as a 2006 format file.  E-mail or Buzzsaw can be used to get a newly saved file back from the original author in a jiffy!