Xion crystal

Xion crystals were a component of TARDISes. If they were jolted out of line, this would cause the passavitor 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)