FAQ: Detect Missing Pages

If you duplicated category and it does not appear on the product edit page, you should reindex all indices. To do that go to: System > Index Managemet

If Detect Missing Pages extension works but does not handle 404 error for some store, check if your store 404 page has unique URL key. Then resave configuration and clear the cache. If everything set up correct, the extension will handle 404 errors ...