Sometimes I run across jobs on Monster and other similar sites that look really good, and they're listed for companies that are good companies that I'm familiar with. However, when I go to the company website and look through their careers pages, the job doesn't seem to exist and isn't listed anywhere on their pages. Sometimes these jobs are posted just today, a day ago, or up to a couple weeks ago.

It seems very strange that a company would list a job opportunity on a site like Monster but have no trace of that opportunity on their very own careers page. Is this a common practice, or should I be wary of listings such as these? What reasons would a company have to list a job opportunity this way?