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The Most Dangerous Bug

There was a thread on Twitter recently from people who caused major problems in software systems, and I was reminded of the worst bug I've ever seen. Long ago I had published this story with a major tech media vendor, but that article seems to have succumbed to bit rot, so I am going to tell that story here. It happened more than twenty years ago when I had only been in QA for a couple of years, but it is seared into my mind... In the late 1990s I worked for a company that handled 911 land line location software for major telcos. So when you choke on that chicken bone and call 911, you can't talk but they send the ambulance to where you live anyway. We handled about 75% of all the telephone numbers in the USA. So from time to time in those days, there would be an area code split, where a new area code is added to a populous area and phone numbers start getting the new area code where they had a different area code before. In the 911 world, we would maintain records for both old