On one side of the nav sphere pitch can only switch between 1* and -1, the other side works just fine going from 1 to 0* to -1*. This can be a problem for when you need to be as accurate as possible.
Reproduce: Grab the nav ball and start changing pitch around the 0* mark on both sides.
It's a small thing, but it's also the kind of thing that can drive you to madness once you notice it.