how can a capacitor fail
Capacitors are meant to hold electrons on one plate (and repel electrons from the other) while NOT passing a current between the plates. If current is allowed to pass between the plates than the capacitor has failed. A capacitor is made up of two isolated plates held apart by a non-conductive material (usually oils). If the oil has been broken down or exposed to a significantly large voltage, electrons will begin to jump the gap between the two plates. At this point the capacitor has failed.