Why indexing MS Word 2007 document corrupts saved file

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Thanks to a pointer from Seth Maislin that auto-indexing an MS Word file can corrupt a document, I can confirm that this is the case.

Reason

Merely adding an index mark into a table of contents, saving, and on trying to open the document, get the error message: “The file cannot be opened because there is a problem with the contents.” The details specify “Unspecified error”.

This error had been bugging me for several years, resulting from auto-indexing a several hundred page document. I knew I could not save such a file, but now I know why.