"I WILL SAY NO..."
...is a pretty categorical statement of "never".
Later, you state:
" But Its unlikely..."
So...it might happen.
And of course, we out here have no idea of the PSU involved here.
There are absolutely crap devices around, that lack the needed protections.
Last Statement sounds like.. Ok..
there are many many reasons why a PSU can kill a motherboard while its plugged in a wrong way.. and there are many many reasons why it cant kill a motherboard... But also i kept PSU safety in mind... so that thing plays a big role why i said No.. it cant kill a psu...
other Reason... When PSU starts... or any Switch Mode Power Supply. Initial Current Spike in very high in primary side of transformer because of High output Capacitance (Output Capacitors).. which is also kept in mid during designing a PSU.. there are certain limits a PSU can detect a short or a very high power hungry device... so if we combine discharged Output caps... with shorted 12v raill.. a Dual Switch Forward PSU of capacity 650 will most likely generate close to 180 amps at secondary for very brief amount of time... which is very high for not triggering OCP....
and he is not usning a 2000-1700 watt PSU.. so that why i said.. OCP will trigger instead of properly frying Mobo.. Its about Probability...
Peace.. I am assuming a lot of stuff... while actual Parameters may be very very different from assumption..
Good Day