PCIe connectors are supposed to be keyed to fit one way...unless, of course, it's forced in.
Well, I could easyly plug them in upside down and burn out S2's stock PSU. It's being repaired now. I never thought that this is possible to do with PCI-E connectors. 
Sorry to hear that. It's really easy to force them in as the keys are nothing more than subtle variations on the shape of otherwise square pinout sockets and are the same malleable plastic material as the entire housing. That's why I always make sure to line up the locking tabs as they are the only visible indicator of proper orientation.