Typically when a fixture like that is connected to a live line it is pretty well grounded. In this case it sounds like the grounding wasn't perfect and pretty m

Typically when a fixture like that is connected to a live line it is pretty well grounded. In this case it sounds like the grounding wasn't perfect and pretty much turned you and the mirror into a voltage divider. Your saving grace was the fact that you were the bigger resistor. If the mirror wasn't mostly grounding out the circuit, the wet skin would have provided nil resistance and you would have probably at least been burned if not killed.