Heading Numbering is simply a List Template in which levels 1 to 9 are attached to styles 1 to 9 (normally, Heading 1 through Heading 9).
Heading numbering is more stable than other types because (as long as you define it correctly initially) it is usually a single list, and it almost never restarts. Where you get problems with heading numbering, remove it and re-apply it. This should re-create it as a single list if it has become broken into pieces.
You have to be looking for trouble to break heading numbering. Unfortunately, converting the document to a different format is "looking for trouble." Converting between Word 6 or 95 and Word 97/98/2000/2001 is thrill seeking, and converting a document to or from WordPerfect is almost guaranteed to break it.
In the default List Templates seen in the Format>Bullets and Numbering dialog under the Outline Numbered tab, three of the samples show the word "heading" in their pictures. These List Templates are automatically associated with the built-in heading styles Heading 1 through Heading 9. By default, Word's built-in heading styles have list levels 1 to 9 assigned. It's this list level that governs the operation of the heading outline numbering.
There are three ways of applying heading numbering: with a style, by direct formatting, or typing. Professional writers tend to apply it with styles, corporate users tend to apply it as direct formatting. Users in trouble sometimes try to correct it by typing it. Any document that has ever been saved to a text-based format or to a format earlier than Word 6 will have hard-typed numbering throughout.
If the numbering has been applied with styles, it is robust and unlikely to malfunction. Because each style can point at only one List Template, all headings in the document are likely to be members of the same list. Text copied from other documents should assume the style definition of the destination document, and thus, become a member of the same list.
When applying heading numbering, it pays to take great care that you use a default List Template that is associated with the Heading styles. These List Templates have special properties: they "know" that they belong on Headings. If you decide to use a different List Template for heading numbering, you must first create your styles 1 through nine, then assign each of the styles an outline level, and finally associate each level of your outline List Template with a different style. It's a lot of work, and it's not worth it: Use the built-in Heading styles: the work has been done for you.
You should apply heading numbering using Format>Style.
Trouble may break out if the user starts fiddling around with the style definitions. They may associate the same style with more than one level in the List Template. A classic example is to have Heading 1 associated with Level 1 (the chapters) and Level 5 (the appendices). This was a recommended work-around for Appendix numbering in Word 6 and 95, so you will often see it in up-converted documents. You need to reset the List Template to cure this.
A rare error, but a far more damaging one, is to have the same style associated with more than one level in a list. You often see this in corporate documents: the List Template is simply cactus: remove and replace it.
A variation on this is that various heading levels are members of different List Templates. This is perhaps the most common spaghetti-numbering error in corporate documents. Both List Templates are now corrupt: you have to remove both, reset both, and re-apply just one of them.
Direct Formatting Headings
If the numbering is applied as direct formatting, it should be applied from the first Heading 1 in the document, all the chapters should be in the same document, and the Chapter Titles should all be Heading 1 style. If the author follows these rules, all will be well. And pigs might fly...
If you apply heading numbering using Format>Bullets and Numbering you apply it as direct formatting, which is not stable. The List Template it is applied as direct formatting, but only to those paragraphs that currently have the built-in Heading styles applied. This updates the document's internal style so that it points to the List Template. However, paragraphs copied in from another document may not correctly accept the numbering, and if the style is re-applied from the template, the headings may lose their numbering, or the template numbering may conflict with the local numbering and corrupt the document.
This can lead to a really nasty situation where you re-apply, for example, Heading 3. This sets the Heading 3 paragraphs in the document as members of a different list.
Problems will occur when text is copied from another document. That text may or may not have heading numbering already applied. Even if it does, it cannot have the same List Template applied. So some or all of the copied text may be in a different outline numbered template from the rest of the document. This is the beginning of the "spaghetti numbering" bug. In spaghetti numbering, one or more heading numbers are out of sequence, and try as you might, you cannot get all the headings to number correctly.
If you catch it early, the cure is simple: remove the heading numbering, save the document and close it. Then re-open it, and re-apply the heading numbering from the first Heading 1 in the document.