"I conducted independent path length experiments to identify the fast and slow axes of all the wave plates and discovered in the process that the specifications provided by the suppliers of some of our QWP'S (Ealing 34-5835) in this regard were wrong. This was later confirmed by the manufacturers (Special Optics, U.S.A.). In my first attempt I did get the wrong sign for the Pancharatnam phase for this reason! I hold a somewhat cynical view towards sign determinations (including my own). Observing the correct sign usually means that the number of mistakes made is even." Page 1163 of [Bhandari, 1988]

Bhandari, Rajendra. "Geometric phase experiments in optics-A unified description." (1990).  Paper shows 1988.