The overwhelming majority of intra-operative hazard situations in tracked ultrasound (US) systems are attributed to failure of registration between tracking and imaging coordinate frames. We introduce a novel methodology for real-time in-vivo quality control of tracked US systems, in order to capture registration failures during the clinical procedure. In effect, we dynamically recalibrate the tracked US system for rotation, scale factor, and in-plane position offset up to a scale factor. We detect any unexpected change in these parameters through capturing discrepancies in the resulting calibration matrix, thereby assuring quality (accuracy and consistency) of the tracked system. No phantom is used for the recalibration. We perform the task of quality control in the background, transparently to the clinical user while the subject is being scanned. We present the concept, mathematical formulation, and experimental evaluation in-vitro. This new method can play an important role in guaranteeing accurate, consistent, and reliable performance of tracked ultrasound.