Have you been using the Document Map on this doc?

If you have:
When you switch into the document map, it applies outline levels to things
it thinks are headings (short, one line paras, usually), regardless of
whether you already have headings.  It been reported in Win & Mac versions
of Word from 97 to 2004, and it's a bug, or a badly-designed feature if you
are in a charitable mood.  Suggested workarounds and a couple aids if Doc
Map just ruined your careful formatting:

Don't use the Document Map, use Outline View instead:
http://www.mvps.org/word/FAQs/Formatting/UsingOLView.htm

On switching into the Doc Map, look to see if a new AutoFormat has been
added to the Undo list.  That's the application of outline levels.  Undo it.
Remember to check for this every time, though it may not happen every time
(this workaround has only been tested on a Mac, and not thoroughly).

After the fact fixes:

Use the ResetPara command on the affected paragraphs to remove all direct
paragraph formatting and reset the para to only style-based formatting (the
doc map adds outline levels as direct formatting).  ResetPara will not
affect direct character formatting (bold, etc) and the usual keyboard
shortcut is control-q (cmd-opt-q on a Mac).

Run this handy macro once posted by MVP Klaus Linke that resets the outline
level of every paragraph to the outline level that is set in the style.

Dim myPara As Paragraph
For Each myPara In ActiveDocument.Paragraphs
 myPara.OutlineLevel = _
   myPara.style.ParagraphFormat.OutlineLevel
Next myPara

If necessary, see:
What do I do with macros sent to me by other newsgroup readers to help me
out?
http://word.mvps.org/FAQs/MacrosVBA/CreateAMacro.htm
(Mac: http://word.mvps.org/MacWordNew/InstallMacroMac.htm)

Hope that helps,
DM