Hard to think of a way in which a TENS machine could cause a severe shock and even mild burns.
Burns require a high current to flow in the victim and a TENS unit doesn't allow that. A battery of small dry-cells power a low-voltage, low-current oscillator that pulses a step-up transformer to create mild electric tingle. (The tingle suppresses other pain by providing a counter-irritant. How well they work depends on the type of pain; anything between good and makes it worse. Well worth trying, but not a guaranteed miracle cure. )
The normal peak voltage is probably between 100 and 200V, but the amount of energy in each pulse is limited by the maximum current of the battery, the drive transistor, and the size of the transformer – all small.
Possibly a combination:
- Unit fitted with high capacity rechargeable or alkaline cells rather than dry cells with high internal resistances. (Allowing the battery to deliver much more current.)
- Condensation reduced the value of a resistor causing the oscillator to vibrate at higher than normal frequency. (Each pulse contains normal energy, but there are a lot more of them per second.)
- The higher than normal frequency happened to be resonant with the transformer causing exceptionally high voltage on the pads.
Although such a malfunction might deliver a surprising extra shock, especially to a damp victim, I'm still surprised a TENS unit caused visible damage. They aren't Widow Makers!
Condensation might explain why the unit is working normally now. Drying out would return the oscillator to it's correct operating range.
If condensation was the cause, the effect was downright unlucky because damp circuit boards are unlikely to work at all!
Dave