A few years back an economic forecaster was asked to explain why his predictions of a 10 per cent return on the Australian equity market that year hadn't come to pass. (The market ended down nearly 40 per cent in 2008). "It's not my fault," he complained. "No-one predicted Lehman."
To which the response is well, isn't that the point? Forecasts are subject to considerable error, due to the tendency for events to screw around with one's cherished assumptions. A plane flies into a building, an investment bank goes belly up, a country defaults, a government changes etc;