Forecasting

Even geniuses don't necessarily forecast well. From Cecil Adams' The Straight Dope:

"A century ago, Daniel Burnham's Plan of Chicago predicted that the city, based on the growth rate at the time, would have a population of 13.25 million by 1950. It didn't come close."

In 2008, about 60 years past 1950, the city has 2.8 million people, the metropolitan area 9.5 million.