More rarely, a webmaster has been in the process of uploading his site when his upload software crashed. This may leave broken links for a short time until the problem is fixed. Generally an ISP will provide, at least for a short time, a redirect page to a new site. And of course some sites simply shut down. This is actually getting more rare as it's often more expensive for an ISP to delete old data than it is simply to prevent updates.
Search engines are notorious for their broken links.
Because of this, all of the above problems can occur, leaving broken links. One of the most debated subject among webmasters is the whole concept of web site awards. These are just graphics that anyone can make up, they are given out by the boatload and they do not prove anything. I beg to differ. Winning an award can be one of the most fulfilling achievements that a webmaster can experience.