There are a number of things you need to do for Word to be even semi-behaved.
Do not use the Normal style. You’re only asking for trouble. Partly because anything you do to Normal is going to affect every single document you create from then on that uses the Normal (which is the default) template. And if you open an old document, it’ll get updated, too.
Next, use the Styles. Never, ever, EVER use anything from the Font or Paragraph groups. And I do mean NEVER. (Did you hear me say never? Damn straight. NEVER.) Especially use them for the numbering and bullet lists. See the thing is, the gallery will only ever show you nine of those, but if you ever start fiddling with indents and margins and bullet styles, it’ll start creating more, which may or may not show up in the gallery. If a style doesn’t exist that does what you need, create it and then use it. Use styles. Never, ever, ever, NEVER EVER use the font and paragraph groups to format ANYTHING. EVER.
If all else fails, copy everything in the document except for the very last paragraph symbol and paste it as plain text into a new document. You’ll have to reformat, but this time you’ll pay attention to using styles, right?
I’ve found that formatting from the end of the document back to the beginning seems to avoid a lot of the issues with list problems.
If you ever copy something from another Office document, make sure you paste as PLAIN TEXT. Don’t paste with the source formatting. Don’t paste with the target formatting. Plain Text, then format it. Using styles. If you’re not sure how to do that, paste it into a text editor first and then copy the text without any formatting.
Did I mention using styles? Use styles for everything. For paragraph formatting, for character formatting, for table formatting, for list formatting (including the numbering).
Seriously, the easiest way to avoid issues and corruption is to only paste plain text and to use styles.
I mean, if you’re just doing a short letter or memo, have at it. But anything longer or more complex, USE STYLES.
This has been a screed written by Morgyn, technical writer, occasional user of Word and Word template developer, who loathes Word with a FIERY PASSION. (And use styles for everything. Seriously.)
2nd ETA: Yes, I know it’s out of order. Deal. Word does not, by default, show you all the styles it has predefined. You have to go into the Styles options and tell it that you want to see ALL the styles, not just the recommended ones. Sort as you please, but I find alphabetical most useful because otherwise Word, as is its wont, will sort it how it thinks you ought to be using them.
ETA: Read Shauna Kelly’s explanation of how to set up lists. It’s a little out of date ('cause she died), but it was written after the Ribbon became a thing and it’s still my go-to reference when I’m forced to use Word for anything I want to lock down.