In November of 1979 someone mistakenly inserted a training tape in to the NORAD early warning system, an error which convinced operators a nuclear first strike by the Soviet Union was underway.
NORAD called President Carter’s national security advisor Zbigniew Brzezinski and informed him that 250 Soviet missiles were incoming, and that President Carter had 5-10 minutes to decide whether to launch our missiles before they were destroyed in their silos.
Luckily, before that decision was made satellite and radar systems were able to confirm that the alert was a false alarm.
Unfortunately, that was not the end of it, as there were more false alarms at NORAD in the following months.
The first half of the following video provides a historical context for the near miss calamity described above. The second half of the video describes the actual event.