When my mother saves a document in Office 2008, she frequently, accidentally erases the .docx extension from the default filename in the save dialog when she names it. Now, you would think that Office 2008 would append the extension anyway like the Windows version does, but it does not. You would also think that the "Append file extension" checkbox in the save dialog would make it append the extension, but it does not.
While the Finder still recognizes the file as an Office document even without the extension, it does not generate a thumbnail or Cover Flow image without it, so this problem causes a significant usability impediment.
(I have tried communicating to her the need not to delete the .docx extension from the save dialog, but she does not remember.)