  One might expect that a quantum undecayed unstable particle (QUUP) should
behave in the same manner as an identical, albeit stable, particle, but it
turns out that this is not always true. We show explicitly that using QUUPs in
the double-slit and Colella-Overhauser-Werner (COW) experiments leads to a
priori which-way information that creates a loss of interference contrast when
compared to the same experiments performed using stable particles. In both of
these cases, a priori path predictability ${\cal P}$ is related to the
interference visibility ${\cal V}$ by the duality relation ${\cal P}^{2} +
{\cal V}^{2} = 1$.
