X-Rated Placeholders -- A random and amusing anecdote arrived from Russell Aminzade <firstname.lastname@example.org>:
I had a job recently developing a Web site for an intranet for a division of a Fortune 100 computer company. One of the things they wanted was a page of links to related materials, so I created the "links" pages with some dummy URLs.
Throughout the Web site, I had used the same placeholder for missing information - "http://xxx" for a server, and "xxx.htm" for the missing documents. My assumption was that the browser would simply present an error to users when that link was clicked.
However, Netscape Navigator [and Microsoft Internet Explorer -Adam] is friendly about resolving improper URLs. It went looking for a site called www.xxx.com. Ooops. I got a frantic phone call from the company asking if I knew that I had included links to pornographic sites on my prototype.
The moral of the story: if you must use a placeholder for URLs, include a real, dummy page on your site, and point those URLs to it.