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If you routinely connect/disconnect with the power on, internal arching inevitably can occur causing the burn marks in your picture. As you can imagine, the instantaneous connection/disconnect, particularly if the conenctor pins are dirty, can arch. I don't know if you do this but I've seen many people do it and, correctly or not, I attributed their failed connectors to that process.
It's always a good habit to first shut down the breaker before disconnecting.
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