No, Nicole Kidman didn't make a fashion mistake at the Emmys. Her shoes were Calvin Klein and they were meant to go together. You just need to look a little closer (which is awkward on the Red Carpet, admittedly)--
Mismatched shoes became a "thing" during the Spring/Summer fashion shows...
So, let's be clear. The shoes aren't exactly mismatched... they're constrasting, but only slightly.
In other words, there are rules, if you want to get it right--