An interesting overview of the system that provides oversight of deployed ICBMs and continuous monitoring of their status (the one that keeps them in high readiness) mentions that the system was created after two accidents in 1967 - in divisions in Perm and Krasnoyarsk (these must be Bershet' and Krasnoyarsk - both have been liquidated). The article gives few details of the accidents, but mentions that "an explosion of missiles" in Krasnoyarsk resulted in casualties.

The accidents must have involved UR-100 missiles that were being deployed at a very high rate at the time. It appears that after the oversight and monitoring system was introduced there were no more (serious) accidents involving deployed ICBMs.