Something like a major telco going out, for example the AT&T 1990 outage of long distance calling:

> The standard procedures the managers tried first failed to bring the network back up to speed and for nine hours, while engineers raced to stabilize the network, almost 50% of the calls placed through AT&T failed to go through.

> Until 11:30pm, when network loads were low enough to allow the system to stabilize, AT&T alone lost more than $60 million in unconnected calls.

> Still unknown is the amount of business lost by airline reservations systems, hotels, rental car agencies and other businesses that relied on the telephone network.

https://users.csc.calpoly.edu/~jdalbey/SWE/Papers/att_collap...