767-300
767-300 IRS alignment only on the ground in a stationary aircraft.
Once released the parking brake on an apron that turned out to be not exactly flat: the couple of inches of movement into the chocks was enough to upset the alignment process.
Turning off 2(!!) out of 3 IRS's leaves you without autopilot capability, which would require you to leave RVSM airspace.
Having only one out of three redundant and critical systems available is a good starting point to decide on a diversion to nearest suitable airfield.
(an IRS with reliable attitude is a critical system, after all it is needed for pitch and power flying, n'est ce pas).