Verification of links is taken care of by a robot of sorts. It is a complex software system that daily checks chapter by chapter, retrieves web addresses and verifies email addresses.
Because of this, most of the handbook's links and addresses are correct. However, several errors are discovered every day. The reasons may be simple, as when a webmaster is moving his files to a new directory, or to a new host. It becomes more difficult when a service is renamed, and appears in a slightly different shape on another host.
Some services just disappear. Maybe the maintainer thought that enough was enough, that there was no fortune to be made of it after all?
Sometimes, a host is temporarily unavailable. Maybe they are offline because of technical problems, a software or hardware upgrade? If the service is great, then these cases are particularly difficult, for how impatient are we allowed to be?
Usually, we try and try for a week or so, before taking it out of the handbook. It may then be moved over to be further checked in a month or two. Rebirths are not uncommon.