Theodore Smith's answer: On this day, exactly 12 years ago (9:30 EDT 1 Aug 2012), was the most expensive software bug ever, in both terms of dollars per second and total lost. The company managed to pare losses through the heroics of Goldman Sachs, and “only” lost $457 million (which led to its d...
Now what was the cause of the bug? Fat fingering human error during release.
There isn't a singular "the" cause usually, and if we do want to press for it, I'd say an aggressive deadline for a major product that needs engineers to slave away was the cause. At that point bugs become stastically inevitable. Whoever decided on that promised deadline was the first responsible person.