From Hacker News:
In 2003, I was working for a startup in India doing GPS/GSM based vehicle tracking system for fleets of trucks…
… my boss asked me to program the unit to give a missed call to the server every hour. If we got a missed call, we knew that unit was still working. In countries like India, giving a missed call is a zero cost way to communicate.
The way I implemented the logic was the unit was to call our server’s modem number every hour at the top of the hour. No random delay nothing. So, soon after that, around 50 units tried to call our server at the same time. Remember the clocks in the units are being run off GPS and they are super accurate. This caused our telecom company’s cell tower BTS to crash. Cell service in my office area, a busy part of Bangalore, was down for a whole 2 hours.