Anomalous Real Parts in theTMatrices of Unstable Particles

Anomalous, possibly large real parts in the $T$ matrices for high-energy scattering of unstable particles are shown to be a dynamical consequence of instability. This effect is estimated to be quite large in the case of the photoproduction and elastic scattering of ${\rho }^0$ mesons from nucleons and can be interpreted as a breakdown of the simple "${\rho }^0$-photon analogy."