from http://bugs.musicbrainz.org/ticket/3421

There should be some sort of URL checking for MusicBrainz Advanced Relations. Perhaps with some sort of reporting system to allow editors to correct broken URLs.

As well as checking for 404 Not Found, it is worth checking Wikipedia URLs to see if they have turned into Disambiguation pages. The MediaWiki API may be of use for checking this: http://en.wikipedia.org/w/api.php