WCAG 2.1 - SC 3.2.3 Consistent Navigation

Navigation mechanism (links, search forms, etc) that are repeated on multiple pages of a site should appear in the same order relative to each other each time they occur, unless the user has chosen to make a change.

Note that this doesn't mean you can't insert additional links within a navigation set-- just that links the links that appear consistently throughout the site should appear in a consistent order relative to each other, unless the user has chosen to rearrange them.

Official Requirements: 

3.2.3 Consistent Navigation: Navigational mechanisms that are repeated on multiple Web pages within a set of Web pages occur in the same relative order each time they are repeated, unless a change is initiated by the user. (Level AA)

How to Fix: 

Make sure your navigation is consistent. One way to accomplish this is to use includes, so that the common parts of your pages are in shared code.