I'm banging my head against this problem now quite some time. For some reason trying to create a depth cube map (or any cube map for that matter) using in-game rendering the gl_FrontFacing in the shaders returns incorrect values. For front faces it returns false and for back faces true which is totally wrong.
I verified everything I could find on the internet. The following I checked so far:
1) culling back faces front faces are rendered and back faces culled (winding order is thus correct)
2) no mirroring is done in the matrices. it's your ordinary camera transformation matrix
What reasons could cause gl_FrontFacing to return utterly wrong values?