It's really hard to forget to renew because most companies send a remind letter when your domain is about to expire. I fully agree with Seeker.[/b]
I disagree. I used to work for web hosting companies -- people would forget to renew their domains all the time. Often, the reminder email would be snared by a spam filter, or simply be ignored. And if the company has moved, and didn't update their address with the domain registrar, they probably won't receive a reminder by postal mail, even if one is sent.

Basically, a lot of people who have web sites really don't understand how the process works. They either assume that the domain name is theirs for life, or that the web hosting company is responsible for it.