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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.
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.
Another thing to look for is aluminum wiring - I don't know if that's "a thing" in your area/house.
AL wiring gets a mostly deserved bad rap, but the worst thing is to connect it to a non-AL-compliant (wrong type) socket and then make it even worse by drawing lots of current through it.
At this point, being disconnected from any wiring, it's difficult to impossible to tell what happened. Clearly there was excess heat.
My first inclination is that there is a problem in the dishwasher itself where it's developed a short circuit and is drawing excessive current. That might account for the damage to the outlet if it was improperly installed or just defective.
I'd check the dishwasher itself carefully before plugging it back in. Chances are there is similar damage in it somewhere.
Also, is it possible you have a circuit breaker on this line that is too large? You should check that too.
This is a classic signature of arcing. This is what arc-fault breakers detect. It happened in one of two ways:
EC&M Magazine made a test rig and took it to four trade shows. Their data was interesting. They tallied under-torqued, torqued within 20% of spec, and over-torqued - and whether a person was an electrician or not. Both groups turned in exactly the same scores.
it was obvious during the test that people who had never made an electrical connection before had as much chance of getting it within 20% as an electrician with decades of experience.
And given the broad tolerance (+/- 20%), it's as likely that the 25% correct were simply attributable to luck.
The presence of "backstab" holes reveals this recep to be a cheapie "Builder grade". They are made to be cheap and fast to assemble, so the builder can get to closing, collect a signature and get paid. When a maintainer has a long-term stake in the reliability of the outlet, they tend to go for "spec grade" or "industrial-grade" outlets. These are in the $2.50 to $10 range, and have better everything - including deletion of the "backstab" connection often replaced by a screw-and-clamp arrangement.
The one thing going for backstabs is you can't screw up the torque setting. But they have so many other problems they are not worth using.
We can only guess, but another reason that happens is simple long-term use by a high-current device. A space heater that draws just under the current limit for extended periods, maybe with a worn outlet, results in slow but significant heat buildup.
I've seen the same thing when extension cords are used for such devices. The plug ends will actually get hot to the touch or melt but the breaker won't trip.
It could be that suds, not water, overflowed the machine and made it's way to the outlet.
Did someone put in the wrong kind of detergent?
Were the wires under the screws or in the push-in “backstab” connections on the back of the receptacle? Backstabs are notorious for failing exactly like this - the contact area can be much smaller than a proper screw connection, leading to heating and meltdown.
Make sure that your new receptacle is using the screw connections with the wire bent into a hook shape that wraps all the way around the screw, and under the screw all the way around - not squeezed out to the side anywhere (unless it’s the clamp type, then keep the wire straight).
