Every time a MOV shorts, its internal structure is changed and its threshold voltage reduced slightly. After many spikes the threshold voltage can reduce enough to be near the line voltage, i.e. 120 vac or 240 vac. At this point the MOV will partially conduct and heat up and eventually fail, sometimes in a dramatic meltdown or even a fire. Most modern surge protectors have circuit breakers and temperature fuses to prevent serious consequences. Many also have a LED light to indicate if the MOVs are still functioning.