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: Old receptacles made of brittle materials like Bakelite can crack under the stress of regular use. Once the plastic housing fails, metal contact points may become exposed, posing an immediate shock hazard.

I walked in and killed the main breaker. I pulled the switch out and held a magnifying glass to the side of the nylon body of the switch. There it was. A crack no wider than a human hair, running from the hot screw to the ground yoke. see electrical crack