Melted terminal block from an oil-filled radiator. A little fault finding at work today. Surprised they hadn't used a ceramic block instead of this plastic one, especially in a radiator. Then again, it was cheap inexpensive. I'll try to find a ceramic block to repair it with.

The cause - most likely poor manufacture. I'd suspect they were missing a single strand, or more, of the copper wire coming into the connector. That can be enough for this to eventually happen. Most common in electrical appliances and fittings that take a large current - almost anything with a heating element: kettle, toaster, iron, immersion heaters, washing machines, and tumble dryers. The latter three being common causes of house fires.

I know, so cheerful! Take care wiring up any plugs, and be wary if they become hot. Once hot enough for any insulation to start melting, the process accelerates very rapidly...