If an operator attempts to send a TARDIS back in time without changing the spatial coordinates there is a chance that the ship could travel back in time and Time Ram itself. Such an event would be known as an Eigen Ram. The Banshee Circuits would activate the Cloister Bell if such a course were entered.
But if the
vortex
shields are not activated when a TARDIS dematerializes it could cause an Eigen Ram without triggering the cloister bell.
In either case if Eigen Ram occurs there will be two time zones at war in the heart of the TARDIS. While lesser species will perceive only a severe distortion of their senses all Time Lords traveling in both the past and future version of the TARDIS will be able to see and interact with each other (almost inevitably causing a violation of the First Law of Time). The operators will only have just over 5 minutes to alleviate the situation before the paradox blows a 30,000 square kilometer hole in the space time continuum. The resulting black hole will be strong enough to eventually swallow the universe. Such an event will, of course, cause the cloister bell to ring. The solution is to turn the TARDIS's Eye of Harmony into a supernova at the instant of destruction using the following procedure.
The Supernova will cancel out collapse of the two black holes merging, causing all matter to remain constant. The Operator will, of course, have to replace the Zyton-7 crystals afterwards.