It’s hard to predict the future, especially when humans are involved. Oftentimes, there are just too many variables to control for to estimate accurately. In the chart above from the Washington Post Wonkblog, Japan’s fertility rate is plotted against various forecasts over the years, and you see forecasts headed upwards, but in reality it decreased consistently since the 1970s. This is based on work from Katagiri, et al (pdf).
See also a similar porcupine chart from Amanda Cox in 2010 that compared US deficit against forecasts.