Invalid hyperlinks, aka broken links, are the bane of webmasters everywhere. I've installed a module that automatically checks looks for broken links and produces a report of any problems. I've also created a rule that sends an email to the webmaster whenever a new broken link is detected.
Check the report regularly and address problems promptly; nobody likes those "404 - Not Found" messages!
On the upside, many "broken" links are actually just redirects because either the destination page has changed or the destination site has set up SSL/TLS, and thus switched all their web addresses from http:// to https://.
