Direct numbering
Numbering can be applied directly to paragraphs, just as a font or indent can be. This is known as direct formatting numbering. If you select a paragraph or set of paragraphs and click the Numbering button on the toolbar, you will get direct numbering. If you use the Format>Bullets and Numbering command on the Format menu, you may get direct numbering, or you may get style-based numbering, depending upon which List Template you choose.
Direct numbering is by far the most common numbering seen in the corporate environment, and overwhelmingly the most troublesome and difficult to fix.
When you apply direct numbering, the paragraph simply contains a pointer to a List Template. If you copy the paragraph to another document the List Template is copied to the other document as well, as a separate object, and added to the ones that are already in it.
Consider a list of seven items. Five were created in the document. They have List Template number 213 applied. Two were copied from another document. Two users edited the other document. The two paragraphs have List Templates 7 and 128 applied. The result now has List Templates 213, 7 and 128 applied to the seven paragraphs. The seven paragraphs are actually members of three separate lists, and only one of them is applied anywhere else in the document.
Similarly, if you move a paragraph from one list to another within the same document, you will get exactly the same problem; and you are likely to get "spaghetti numbering" as soon as you do so, as a result of the mixture of List Templates present within a single sequence of consecutive paragraphs. (to give the simplest example, you might end up with a list numbered: 1,2,5,3,4).
If other users apply or adjust any of these paragraphs, they will apply their own List Template to some of them. Nevertheless, chances are the numbering will break before that happens.