In that case, your mailmerge main document may have acquired some of corruption. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it. With a mailmerge main document, you'd also need to re-connect to the data source and re-apply any filtering.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

This is a long-standing problem, more than a year. I've created many mailmerge documents, including the barest single-line test documents, and all have suffered the same problem.
Just to put this in perspective, if I close the to-be-merged document before the merged document, there's no problem, and I have gotten pretty good at remembering to do this. I'm simply curious about what might be causing the crash. Have you ever heard of such a problem before?
I don't know if it's relevant or not, but I use Dragon Naturally Speaking (Medical Practice Edition) for dictation, for which there is an add-in in Word. I do not use any other Word add-ins.
Thanks again for your help on this.
Reid