It rarely happens but sometimes a breaker malfunctions and won't trip. They are designed to fail the other way and trip at lower amps rather than higher. Your problem where the connection fused had to be a bad connection with a corroded prong or if it was wet that could also cause it to arc. When a connection arcs just a little it causes more resistance which causes even more arcing and heat. Then the connection fuses and actually becomes a better connection. All this can happen without tripping the breaker. But remember, the cord is protected by the device you plug into. I suspect that there was a problem with whatever you plugged into. That is what should have tripped.
RV- 2014 Thunderbolt XLR 386X12 Called "The Castle"
TV- 2013 Ram 3600 DRW Diesel Called "Hercules"