In reply to Manuel.

copy&paste mistakes happen to everyone.

Exactly. This is the main reason to add that check.

In its first iteration it was using the server as proxy to follow each URL. However this didn't work that well. At the end only a regex "well-formed" check remained. This still catches the more common copy/paste errors but not things like 404s. See the trac ticket (linked in the post) for more info.