All it would take would be a particularly threatening NATO military exercise coupled with mobilization on both sides and a particularly paranoid General Secretary to get the Russians to preemptively attack.
Indeed, this almost happened in 1982 during the NATO exercise 'Operation Able Archer'. Apparently there was an assumption in the Soviet military echelons that any attack by NATO would come during a publicly announced training exercise to lull the Warsaw Pact into a false sense of security. Cooler heads did prevail, but we have to keep some perspective, it's not as if the US military was immune to the same mentality as well, since like I said by far and away the vast majority of invasion paranoia was on the NATO side of the Iron Curtain.
This was what the 'Hot Line' telephone link between the White House and the Kremlin was for, after all. Should there be some erratic behaviour by detected by one side, all it has to do is phone up the other side to check what's going on. Although it's funny that neither side used this to their own advantage:
"What are you people doing!? Our radars are going crazy with NATO activity!"
"No, no, no, there's nothing to worry about! We're just testing logistics and conducting communication trials, that's all."
"But...but...the USAF is bombing Prague!"
"Nah, your electronics must be acting up, mate."