Webhosting customers are messies, at least some of them - or (sadly, that's the truth) the bigger part of them. Some people still think they can run the same blog software or CMS for years without ever caring about upgrading. I tell my customers over and over how important it is, to keep their website up-to-date and don't let any outdated code lying around. Still, as long as their website doesn't get hacked or defaced, they don't really seem to care.
If you're in the same situation as me and you are providing webhosting services to your friends or customers, read on.