I'm not convinced. A clock/radio might take 5W at 0.7pf i.e. 31mA load via the faulty contacts / connections. If their resistance reached 1 kilohm it would cause a heat dissipation of around one watt, which would warm the pin but isn't enough to set fire to anything. Have you ever seen an actual burn-out, caused by load rather than flashover, on a circuit carrying a small fraction of an amp? Scorched plug on a table lamp? Theoretically possible but very, very unlikely. By the time contacts get so loose / corroded that they could heat up with that load, they usually go open-circuit completely or so intermittent you know there's a problem because you can't get the load to stay on for more than a second at a time.
If it was a rewireable plug, I would be more inclined to think it was badly wired, there was a stray wire strand just out of contact. The flex moved, the short occurred, drew an arc which charred the plug and the heat was conducted up the cable.
If not, and/or the device was not of a reputable brand, then the cable may be substandard and the insulation broke down, and the arc spread to the plug.
Or, there's some other totally unrelated factor that you haven't been told about! I agree there's no particular reason for the RBCO to have tripped. Sharp, detailed pics would be very interesting...