Identify the apparent statistical mistake in this commentary :

The median cost of a house in [Duarte, California] is a whopping $4,276,462, making it the most expensive housing market in the country. It ranks No. 1 on Forbes’ annual ranking of America’s Most Expensive ZIP Codes…. [O]nly 12 homes are currently on the market. So a single high-priced listing (like the mammoth nine-bedroom, built this year, that’s selling for $19.8 million) is enough to skew the median price skyward.