PSU guru Jon Gerrow (aka JohnnyGuru) working at Corsair these days claims that improper plug-ins are to blame for the 12VHPWR melting issue.
His attempts to melt the connector using other methods proved fruitless. Several media claim that poor solder points contributed to the melting process. However his experiments disproved it. Because of this, Jon has deduced that it's hard to get those plugs all the way in, and sometimes it looks like they are in already when they are not. The plug is not secure until the clip on it clicks into place.
Three PCIe cables were linked to the NVIDIA 16-pin adaptor to a power supply. He gathered a number of 12VHPWR connectors and demonstrated that one of them, despite being severely damaged (by himself), did not go beyond 52 ° C and hence could not replicate the issue. According to JonnyGuru, the 16-pin connector cannot be fully inserted because it is not simple to attach and can be quite challenging if you are not familiar with what you are doing. Neither Nvidia nor the other board partners have replied to the concerns raised.
The truth is that everything we have seen on this subject remains inconclusive thus far. Nvidia is "still testing".
Read up on his findings here.