MS Office is just not compatible with itself
For all those saying, stick with a known format like MS Office, I get as many, if not more people, with a problem where different versions of MS Office have corrupted a file. If you're in a corporate environment, with constant upgrades to the latest version, it mostly works. But if you're stuck with an ancient version because of personal reasons of cost, or hardware issues, or maybe in academia with a wide diversity of versions when sharing files - MS Office is just not compatible with itself. I've been asked to fix a number of broken Word documents, that have been passed around between people, which have gone bad. Unreadably bad. Tracked changes seems to be one major trigger. The fix? Import them into LibreOffice, it is sufficiently resilient to recover many MS fuckups.
Biting the hand that feeds IT
