tedster - 6:38 am on Aug 17, 2010 (gmt 0)
A false positive - as in a heuristic for the algo tries to catch some practice in order to automate a type of penalty. But sometimes the heuristic test flags a few sites by accident that were not actually doing what the heuristic was trying to catch.
For a classic from the early days of search, a page might have a white background and also use white text inside a div somewhere. Now that div happens to have a dark colored background image so the text is not actually hidden from visitors. But the early days of search, attempts to catch hidden text in an automated fashion sometimes flagged this kind of thing as hidden text - and that is a false positive.
There was a publicized case on the Google Forums last year where someone got penalized because Google's page layout simulation showed "too much white space" - but that assumed white space was actually filled by an iframe.