Year 2000 problem

The failure that mostly did not happen, which is exactly why it belongs here. Decades of software stored years as two digits to save memory, so 2000 would read as 1900 and break date arithmetic. Worldwide remediation cost an estimated $308 billion, and the problems that did appear on 1 January 2000 were generally regarded as minor. People still argue whether that proves the money was wasted or well spent, and that argument is the lesson: successful prevention looks like nothing at all. Y2K also showed how long a shortcut can outlive its reasons. The next one is already scheduled, in 2038, when signed 32-bit Unix time runs out.