What is happening is there is a very specific bug in Office 2010 focused around equations and inserted images. Most of the problem has been fixed by Office 2010 SP1, but that doesn't help existing files. For some reason Word just forgets to put one of the
XML tags back in the document. You've seen the resulting error. Unfortunately, while the fix is simple, just typing in the missing tag, figuring out where to put it based on the error message is nearly impossible.
Here are the general links on the topic:
Can't open Word File because of end tag/start tag mismatch error...need XML help -
MS Fixit tool: http://support.microsoft.com/kb/2528942
Try the rebuilder created by Tony Jollans:
http://www.wordarticles.com/temp/Rebuilder.dotm
I understand that the Upcoming SP1 will fix this problem for new documents. KB 2528942 provides steps to fix problem in existing
documents.
Discussion by many affected people. a couple in discussion are also fixing problem if Tony’s fix doesn’t
work: **http://social.answers.microsoft.com/Forums/en-US/wordcreate/thread/581159d0-9ebc-4522-b30c-53e33e8268e1**
Now that you have been "bit" by this bug, you may want to use these macros to setup a "true autosave" that does a file save as at set time intervals appending the timestamp to the file name. That way, when there is another problem, you can go back