Reports (e.g. by Ed, a contributor to this group) suggest that this can
occur when
a. the merge is automated (e.g., run from an Access application)
b. Word is not visible before the merge starts

Ed's suggestion was that the automation code should
a. make the Word application visible (this actually helps fix a slightly
different problem)
b. activate the Word application
c. do the merge
d. activate the Word application (or perhaps document - I'm not sure)

Peter Jamieson