There's a few explanations for the mismatch of PR and site quality; and some are black-hat SEO techniques. Normally when this happens, it's because of some kind of large
First, a server redirect for search bots to go to a different site (Probably much higher quality).
Second, PR takes a while to impact. The site could have degraded significantly in the past few weeks due to a change in management, domain sale, etc.
Third, the PR resource you looked at might not be 100% accurate (or a paid site).
And fourth, only for objectivity, your perception of a 'high quality' site isn't the same as what Google or Yahoo or Bing! thinks.
There are many explanations for this kind of behavior, some are blackhat, others are just mistakes.