Taxing along in LHR last week I heard an excited and somewhat vexed antipodean voice calling out ATC over some minor stop bar issue twice, seemed to be from a CX. ATC man calmly informed him that the verbal clearance had overall priority in any case. No biggie, it just seemed an over the top reaction in a very well controlled airport but I couldn't help thinking what a long flight that must have been for the other mortal pilot.