False alarms leading to nuclear war make for interesting movie plots, but when they happen in real life, they are nightmarish, at best.
One such nightmare happened on the morning of November 9, 1979. NORAD computers alerted personnel to a massive attack by the Soviet Union. Data showed a barrage of incoming nuclear missiles launched from the Soviet Union and its submarines against the United States.
Responding to the apparent attack, military commanders scrambled interceptor fighter planes, ordered the President’s “Doomsday Plane” into the air, and prepared for a retaliatory launch of nuclear missiles.
Before matters could escalate beyond the point of no return, computer operators discovered that a technician had loaded a test program but failed to switch the system into “test mode” before running it. The situation was corrected, but not before raising alarm throughout the defense community.
While the 1979 incident would not repeat itself, NORAD was not immune to further false alarms. Three more scares occurred within the next year, each caused by faulty computer chips. In one of these incidents flash traffic was sent to US Air Force command posts around the world, advising that a nuclear attack was in progress. source
These errors were not limited to the US side of the ocean. On September 26, 1983 Soviet Lt. Colonel Stanislav Petrov was in command at Serpukhov-15, a bunker where the Soviets monitored their satellite-based detection systems. Shortly after midnight alarms went off, indicating the launch of five US ICMBs toward Russia.
Petrov did not have time to get confirmation of the launch before having to report to Soviet high command. He went off of a hunch that it had to be a false alarm. He reasoned that a US first strike would come in the form of a massive attack, rather than just five missiles.
As it turned out, he was right. A faulty Soviet satellite picked up the glint of sunlight off the clouds near Montana and interpreted it as a missile launch. Petrov’s hunch prevented a retaliatory launch by the Soviet Union. His actions remained classified until the end of the Cold War. source
Sometimes the “whoops” had nothing to do with either side, but was the result of a third party. On January 25, 1995 Norway launched a Black Brant XII rocket from the Andøya Rocket Range in northwestern Norway. It was a scientific project, designed to study the aurora borealis.
The scientists had notified thirty countries, including Russia, of the planned launch. This information did not make it to the Russian radar technicians, however. Upon launch the rocket was spotted by an early warning radar station in Murmansk Oblast, Russia. On radar it appeared to be a US submarine-launched Trident missile, and its trajectory corresponded with what a radar-blocking EMP missile attack would look like, immediately prior to a massive nuclear attack.
Russian military chain of command went into full alert, and Russian President Boris Yeltsin opened the nuclear briefcase in preparation to authorize a nuclear response, if necessary. With ten minutes available to decide whether to authorize a nuclear response, radar operators used eight of those minutes before concluding the trajectory did not place Russia in jeopardy.
This incident was the first and only time a nuclear weapons state had its nuclear suitcase activated and prepared for authorizing a nuclear launch. source