A Reddit user posted photos of his 12VHPWR that melted. You can see from the photo it's an original Nvidia adapter that ships with RTX 4090 graphics cards.

The story comes from the NVIDIA subreddit via redditor u/reggie_gakil. The card used is a Gigabyte GeForce RTX 4090 Gaming OC. The Reddit user claims that he was playing Red Dead Redemption 2 when he senses smoke. Typically 12VHPWR adapters appear to be particularly susceptible to cable bending around the connector, which the user may have done. In contrast, the pictures suggest that this is not the case. It appears he was utilising the 8-pin to 12VHPWR converter provided. PCI-SIG has warned that gradients like this can quickly get dangerously hot. However, 12VHPWR should be able to supply power for up to 600 watts, while the RTX 4090's TDP is 'only' 450 watts.

There have now been two complaints of melted RTX 4090 16-pin adapter cables, the other from a user with an Asus RTX 4090 TUF Gaming OC card.  A representative from Nvidia has reached out to these users. 

NVIDIA - "We are investigating the reports. We are in contact with the first owner and will be reaching out to the other for additional information." 

NVIDIA RTX 4090 12VHPWR plug melts and burns during gameplay