  A wrong derivation of the phase of a propagating massive particle which has
repeatedly appeared during the last years has the advantage of leading once
more to the important question of the phase difference between the mass
eigenstates constitutive of an oscillating neutrino described in the plane wave
formalism. Serious errors of principle are pointed at in a number of simple
calculations and the oscillation length is derived in a way which allows to
show that the standard result can suffer but minute variations in the
ultrarelativistic case.
