There's a great documentary ''Countdown to Zero'' that talks about just how close the US and Russia come to mutual assured destruction. The closets it seems(Well this is what I remember from the doc anyway) was when US aircraft triggered the Russian alert system into thinking it that the US had fired nukes(Of course the US had no idea that this would happen)and the Kremlin's generals marched into Boris Yeltsin's office and demanded a response. Yeltsin simply refused to believe the Americans would do such a thing, and decide not to act thus saving the world.
Cheers Yeltsin.
There is more than just that occasion. The one you are thinking of is the Norwegian missile crisis where a scientific missile was launched that just happened to take a similar path and trajectory to what the Russian detection systems would have expected a Trident missile to take. I'm aware of at least two other similar incidents.
Stuff like that suggests to me that the whole idea of MAD as a deterrent is just as likely to result in an accidental escalation as it is to result in long-term peace. If it keeps on going long enough then eventually one of these incidents will happen where the person with their finger on the trigger does not think as logically as Yeltsin, Petrov or Arkhipov did. If that happens, I'd rather us not be one of the nations that has nuclear weapons. The UK as a nuclear armed ally of the US would have almost certainly been one of the targets if Russia's dead hand "fail-deadly" were to be activated.
That's in addition to the stuff I've already talked about with regard to my opinion that MAD is getting less and less viable as more countries gain nuclear capability and technologies that would make it harder to identify an aggressor such as submarine launched ballistic missiles become more prevalent.
There is an interesting article by George Shultz, William Perry, Henry Kissinger and Sam Nunn from a few years ago that talks around this subject and one of the things it says is that MAD is too unstable as a deterrent. It's here if you want to read it: http://www.wsj.com/articles/SB10001424052748703300904576178760530169414