So, I've experienced this first hand today.
it was my friend's pc.
at first there was this smell, then there was smoke and then the room filled with smoke..
disconnected power as soon as the smoke appeared but it was already too late.
all cble attached to the mobo was melted.
the sata cable and even the front panel audio header.
surprisingly 1 RAM and the HDD survived. even though the sata cable melted on mobo end.
I hope the processor can be re-used.
the psu is about 1 year old.