1. An implantable cardiac device comprising a plurality of electrodes configured for implantation into a patient for at least sensing cardiac signals and operational circuitry electrically coupled to the electrodes, wherein the operational circuitry is configured to perform a method comprising: sensing a signal with the electrodes; detecting events from the sensed signal; establishing parameters for characterizing detected events as wide and narrow; capturing a series of detected events; characterizing the series of detected events as wide or narrow; identifying a qualifying narrow-wide-narrow sequence of detected events; and determining that the wide event of the qualifying narrow-wide-narrow sequence is an overdetected event.