The forum navigation menu will show a different number of menu items than the rest of the site if you change the number of elements in the navigation from the default number.
I have my settings to show 16 navigation elements to members and only 12 to non members. When a signed in user clicks on the forum tab, the page will reload with only 12 navigation menu elements. This needs to be fixed to matchup with the rest of the site and show 16 (or whatever number of navigation elements is assigned to members).