  We discuss the disturbance by measurements which unambiguously discriminate
between given candidate states. We prove that such an optimal measurement
necessarily changes distinguishable states indistinguishable when the
inconclusive outcome is obtained. The result was previously shown by
Chefles~[Phys. Lett. A 239, 339 (1998)] under restrictions on the class of
quantum measurements and on the definition of optimality. Our theorems remove
these restrictions and are also applicable to infinitely many candidate states.
Combining with our previous results, one can obtain concrete mathematical
conditions for the resulting states. The method may have a wide variety of
applications in contexts other than state discrimination.
