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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.
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)
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.