In 1965 Royal Dutch Shell put into service what it called the Unified Planning Machinery (UPM), a computer-driven system meant to bring more discipline to the company’s cash flow planning. This kind of rational, model-based financial forecasting was very much in vogue in the 1960s. But before long, Shell’s top executives realized that many of the commitments they had to make extended well beyond UPM’s six-year time horizon—and that even within that horizon, UPM tended to get a lot wrong. In the early 1970s they shut it down.

A version of this article appeared in the May 2013 issue of Harvard Business Review.