I have a large document in Word 2003 (over 300 pages) and now twice the
document map has been corrupted with thousands of random entries created that
shouldn't be there (random in the sense that they are not headings but
seemingly random lines of the body of the document).

The first time I had to go back to an earlier revision of the document
because I could not figure out how to fix it - which of course meant I lost
all my changes. But now something has caused the exact same thing to happen
again.

If I select an erroneous item in the document map and then go to the styles
and formatting pane and then click on 'select all n instances' and then
click 'update to match selection' on the same style, then it fixes some of
them but not all of them and then other random ones start appearing again.
Arrrrgh!