My dishwasher was plugged into this socket, and then this happened, tripping the circuit breaker.
So I assume there was a short in the outlet, but how could that have happened? I replaced the outlet and plugged the dishwasher back in, and all seems to be working well.
There was no sign of water in or near the receptacle, and I can't imagine a piece of metal or wire somehow got into the wrong place. I'm now concerned it might happen again, but I have no idea where to check.
Best Answer
The most likely explanation is that the screws were loose causing a poor connection, and arcing under load - evidenced by the fact that the base material has gone brown where it's been hot over a long period. What happens in this case is that the contact surfaces slowly carbonize (so become higher resistance, and get hotter), and it gets progressively worse
You can see that the right-hand screw-terminal has actually moved because the plastic melted. I assume that it moved enough for the contacts inside to either come into contact or arc between themselves, and that's what tripped your breaker.
It's unlikely this will repeat itself, provided the new outlet has been wired correctly, but it wouldn't hurt to keep an eye on it for a few weeks, just to be sure. Also, as mentioned in the comments above, check the plug and replace if there's any evidence of damage.