IT both drives and implements the modern Western-style economy. Thus, we regularly see headlines about staggeringly large amounts of money connected with IT mistakes. Which IT or CS decision has resulted in the most expensive mistake?
Not long ago, a fair number of pundits were doing a lot of hand waving about the financial implications of Sony's troubles with its PlayStation Network, but an event like that does not count here. In my school days, I talked with an inspector from The Guinness Book of World Records who explained that for something to be "a true record," it could not be a mere accident; there had to be direct causation starting with human intent (i.e., we stuffed 26 high school students into our music teacher's Volkswagen Beetle and closed the doors).
Sony (probably) did not intend to see how big a mess it could make with the least attention to security, so this and other such examples of false economy will not qualify. Another candidate could be IBM's choice of Bill Gates over Gary Kildall to supply the operating system for its personal computer. The damage from this decision is still accumulating at breakneck speed, with StuxNet and the OOXML perversion of the ISO standardization process being exemplary bookends for how far and wide the damage spreads. But that was not really an IT or CS decision. It was a business decision that, as far as history has been able to uncover, centered on Kildall's decision not to accept IBM's nondisclosure demands.