Probably a loose connection. Either the blade was worn or the receptacle was worn, or even both.
It could be (less likely) a loose connection at the terminal. But that usually just results in loss of power to the receptacle slots with the burning showing at the terminals.
You might be surprised that copper has one of the highest thermo-conductivities of most all metals, a loose connection at the screw or back stab can very easily cause this kind of damage long before you loose the connection at the screw, most of these I have run into are the ones advertised as being made by the Amish but in reality is nothing but a fancy wood cabinet around a 1500 watt electric space heater, which will find the weakest link in any electrical system, and yes they are advertised as to keep the main heat source from running as much but in many cases if it is located by the main thermostat it will keep the main heat source from even running which then allows this heater to run full time, as I have posted before, if I know that a certain receptacle will be used for a space heater I will make sure the wires are pigtailed and the wires to the receptacle are run around both screws like an "S" so that they also tighten as the screw is tightened, this gives allot more contact area of the connection and even with a 15 amp receptacle have never had one fail since, but I would make sure that the cord cap was also not heated to the point of discoloration as this is a sign that it also has been damaged and needs to be replaced.
Here with the cost of heating with electric over gas you will be paying a much higher electric bill than if you would use just gas, but the sellers don't tell them that part and just claim it will save them so much money which is the second most call back we get when they do get their utility bill and are shocked by the high electric part of the bill and I have to sit down with them and do the math so they can see that in no way can these heaters save them money, some went up as high as $400.00 a month over just using their gas fired furnace.