I'm starting to wonder about Seasonic. My 650w PSU can handle two EPS cables but comes with one, and extras are only available through expensive third parties - Threadripper NEEDS two cables. So... I give in, and order the 850w version of the same PSU. It comes with two cables. Perfect, right? Wrong!
I spend the time to pull the PSU cover from my case, disconnect and pull the old PSU, then install and connect the new PSU and reinstall the PSU cover. I flick on the PSU, and there's a brief flash from the motherboard. Nothing unusual so far. I hit the power button - nothing. I look and realize I missed plugging in the PCIe cable to my card, but I'm still suspicious - the computer should have at least tried to fire up. I plug it in, try again. Nothing.
After probably about an hour, I am able to figure out that no, my motherboard isn't dead - but half of one of the EPS cables is... probably wired incorrectly. The motherboard has one 8 pin EPS connection and one 4 pin EPS connection - if the bad half of the one cable is plugged into the 4 pin connector, or either side of the 8 pin connector, no-go - nothing. Plug the other side into the 4 pin connector, and the other cable into the 8 pin connector, everything is great again.
I think this may be my last ever purchase from Seasonic!
Edit: I don't think that half the EPS cable in question was just dead - the computer will still at least try to boot if it's short a connector - plugging in the 4 pin connector in question completely prevented the computer from doing son.