MOVs have finite life expectancy and "degrade" when exposed to a few large transients, or many small transients. Every time an MOV activates (shorts,) its threshold voltage reduces slightly. After many spikes the threshold voltage can reduce enough to be near the protection voltage, either mains or data. At this point the MOV conducts more and more often, heats up and finally fails. In data circuits, the data channel becomes shorted and non-functional. In a power circuit, you may get a dramatic meltdown or even a fire if not protected by a fuse of some kind.