this problem is age old as the original MOLEX connector. the connectors inside that plastic shielding often float, and are not necessarily inline with each other even if simply held in your hand and yet to be connected. remember connecting old CDRoms, old Floppy 5.25 drives, or of course original Case Fans, at times even daisy chaining these atrocious space cluttering connections together, you can glance and see some pins with lesser coupling contact than other pins, all within the same molex connector. but no one realises this anymore today, it's all plug and pray, right? now with these behemoth GPUs requiring a stretching of said molex or PSU lane wires, to barely fit at a cases' end and pivoting it to kingdom come, what could go wrong? let a professional who understands connecting these together at a certified authorized computer store build if for you, peace of mind. some professionals I know aren't even touching this generation of cards at all. and here we thought PSU makers together with Nvidia were going to have a new connector solution with so called PCI5 or ATX 3.0 , it's just as bad for crying out loud. do away with floating molex wires, and get back to the drawing board, or best yet, these fugly graphic cards should REALLY have an EXTERNAL power supply to attach as a console does. and include a powerbar to filter line noise.
slightly OT, but if you wired an AT (yes, pre ATX) powersupply incorrectly, your electrical panel would fly out of the room.