[sparky]

To be clear, my differential pairs are impedance controlled in the sense that their trace/space yield the correct single-ended and differential impedance values at the nominal dielectric permittivity and thickness, but OSH Park doesn't additionally guarantee permittivity and thickness tolerances to guarantee that my nominally-50-ohm traces will be actually-50-ohm traces +/- 10%. That said, I've run the numbers (using the excellent and free Saturn tools [1]) for the stated Er range of 4.2 +/- 0.3, and a little variation in the nominal 7.8 mil thickness, and things still looked OK.

Fingers crossed!

[1] http://www.saturnpcb.com/pcb_toolkit.htm