Xion crystals were a component of TARDISes. If they were jolted out of line, this would cause the passivator to go into flux and leak coolant into the aethiopathic chamber — meaning that the positioning system would overheat. This happened to the Doctor's TARDIS when the Tenth Doctor dodged the Erewon Armada. He made repairs when he manipulated the ball of a giant pinball machine into striking his ship. (COMIC: Pinball Wizard)

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