You would do well to go read up on the 1990 AT&T long distance network collapse. A single line of changed code, rolled out months earlier, ultimately triggered what you might call these days a DDoS attack that took down all 114 long distance telephone switches in their global network. Over 50 million long distance calls were blocked in the 9 hours it took them to identify the cause and roll out a fix.

AT&T prided itself on the thoroughness of their testing & rollout strategy for any code changes. The bug that took them down was both timing-dependent and load-dependent, making it extremely difficult to test for, and required fairly specific real world conditions to trigger. That’s how it went unnoticed for months before it triggered.