It seems that you could force the mobo eight-pin power lead into the PCIe one, or maybe it was the other way around.
This would lead to a smoky death for all components involved, peeved users because it is never their fault, and, in general, universal disharmony. What is the solution? Redesign the connector to it is harder for the Neanderthal twits to do this, apparently.

As you really can't see in this blurry picture, the mobo eight-pin is on the left, the new PCIe eight-pin is in the middle, and the six-pin older one is on the right.
Compare that to the older model (below), and you probably won't see a difference unless you look really hard. I could not find both sets in the same booth, the above one is from an upcoming Hiper PSU, below from a Thermaltake at CES. µ
