Incredible story: In 1971, a tiny bug in AT&T’s switching software caused a cascading failure that took down nearly all long-distance calling in the US. The entire system collapsed because one switch sent a malformed signal and every other switch copied the same failure. Wild early example of distributed-system fragility.

I'll bet it is, but that link is a 404.

Yeh I can't find anything about this, only the outage in 1990:

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