  A linear 50/50 beamsplitter, together with a coincidence measurement, has
been widely used in quantum optical experiments, such as teleportation, dense
coding, etc., for interferometrically distinguishing, measuring, or projecting
onto one of the four two-photon polarization Bell-states $|\psi^{(-)}>$. In
this paper, we demonstrate that the coincidence measurement at the output of a
beamsplitter cannot be used as an absolute identifier of the input state
$|\psi^{(-)}>$ nor as an indication that the input photons have projected to
the $|\psi^{(-)}>$ state.
