I know this is frustrating. My experience is, that there actually is something in-between coded as a headline and Word is taking its cue from it. I never working in 2000, but I can suggest the following:

First, try removing all formatting, paragraph and character from the faulty heading, adn then reapply the heading style. Sometimes this works.

If not, and if you can turn on hidden characters in 2000 (showing the paragraph marks, word spaces, etc.), do so and see if there is an empty paragraph you might have missed. Open "Normal view", if 2000 had it, and look for a page or section break that might be "hidden" in the page layout view; it could have the heading style applied.

If you find neither of these, I would try selecting everything between your last prpoerly numbered heading and the faulty one, and remove all formatting, both paragraph and character, before reapplying the proper styles fot boddy text, etc. Depending on what and how much comprises that in-between, you might wish to do this one paragraph at a time. Be sure to select the entire paragraph including its completing paragraph mark when doing this, reapplying the proper style afterwards and then any local formatting (a word in italics, for instance).

Hope this helps.
Ulodesk