Fair enough - as you say, (virtually) nothing is impossible.I was once in the position of having to approve acceptance of an externally designed power supply for a somewhat specialised application. I had to travel down to the south coast to see it working. Everything seemed great, but I asked them to show me what happened when the input voltage dropped. When smoke and flames stared to pour out they hurriedly left the room. OK - such a product would hopefully never reach the market, but it has always made me think that nothing is impossible.
However, that does not alter the generality of what I said. The experience you describe must be incredibly rare - I think I would probably struggle to deliberately design something so that it would burst into flames if the supply voltage to it was reduced!
In any event, even if such were possible, I don't see that it would help in terms of the BS7671 regulation about VD - since, particularly for a sockets circuit (into which anything could be plugged) there's no way that a designer of an electrical installation could know that such a vulnerability might exist in some future loads.
Kind Regards, John