It's also an excellent way to check for security problems, for pages which should be generated just for you. Increment/decrement the number - get someone else's information. I have reported countless instances of this with varying rewards and success; you can count on someone, somewhere making this mistake regularly.
Yeah, makes me think of the owasp top ten, though this site's problem isnt really insecure direct object references (as all the postings are public) more like "making it easy to infer information about the target."
