See also "Swiss Cheese Accident Model" and Melted Cheese Accident Model.
Also see also Stockton Rush's statements on safety. He said that most accidents are caused by operator error, so making the sub strong enough wouldn't affect safety.
Molten cheese means that something that was intended to increase safety became a hazard during an incident.
Think of how the SCRAM at Pripyat/Chernobyl caused the reactor to go supercritical because of the graphite tips. The control rods should have reduced reactivity in the reactor, but the first section of the rod increased reactivity.
Or how a hard hat dropped from a height may cause injury.
