When I log into either Meta Stack Exchange or Stack Overflow and then log out again and I end up at a 404 page.
Tried on Chrome and Firefox with the same results.
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OK, it is a bug... but I think it will be a no-fix.
You end-up on that specific link because the logout redirects to the last page you viewed. As you are participating in the new navigation test group you end-up as an anonymous user on the new navigation page. The anonymous user didn't sign-up for participating in the new navigation and therefor a 404 is rendered (which is by the way the default behavior for pages you don't have permissions for).
If you click on a question first and then hit logout you'll be fine...