  Quantum interferences between non-orthogonal states are the best
approximation of a joint realization of the non-commuting physical properties
represented by the two states. As I have shown recently, such interferences can
be used to demonstrate that quantum physics deviates from classical causality
in the limit of small action. Here, I point out that this proof constitutes a
failure of the principle of least action and consider possible implications for
our understanding of fundamental physics.
