I'm using mail merge only to fill data fields within a report, and only one "record" is being used (in other words, it's a single-recipient mail merge with many mergefields). The problem is that the section numbering appears to be fouled up during the mail merge process.

I've fixed the problem, however, using the "List Styles" feature for section numbering. I had to switch some headers (toggle between, say, Heading 3 and Heading 4) to get the numbers to update properly, but that's a minor problem.