I have the same problem. It only occurs when working on large documents. Some of the authors of the document use PCs, so some editing may have occurred in a different version of Word. The smallest file on which it has occurred is 40 pages, 11MB containing lots of Track Changes editing and many illustrations. The problem occurs if the file is saved on the internal HD or an external HD. It seems to me that the problem must be with Word because it has occurred on 3 different MBPs (including a retina display). On each machine Word has been reinstalled (on more than one occasion) and is routinely updated. The only solution is to turn off the computer. All editing that has occurred since the last Save is lost. I now manually save the file about every 2-3 minutes and save it as a different name about every 15 minutes. That's obviously not efficient from the perspective of HDD space (one day I ended up with over 30 versions of a document that was almost 20 MB) but it's better than losing a lot of editing. Any suggestions would be appreciated.