Think Good:
Another good reason not to "back stab" receptacles.
Looking close...is that a little bit of green corrosion on the black conductor??
Looks like THW insulation, metal box, so a guess it's BX. Self-grounding receptacle (look at the top yoke), appears to be "newer" as it's probably been replaced.
Thinking further, the wire may have been bad, or starting when the device was replaced. Finally, the circuit may be over-fused, and/or having a "heavy" load on for extended time. (Electric heater, cooking appliances, etc)
Lastly, also could be excessive "heat" from the male attachment plug.
John
