I've noticed that when a website links to a particular page on my site, then instead of displaying the correct page it will give an error announcing that the page is forbidden. The page will display correctly if the URL is copied into the address bar instead.
If you are seeing this page instead of the one you expected to see, then this may have happened. If after trying the method given above, you still can't see the page you are after, then it may have been moved or deleted.