  A simple consequence of the angular momentum conservation in quantum field
theories is that the interference of s-channel amplitudes exchanging particles
with different spin J vanishes after complete angular integration. We show
that, while this rule holds in scattering processes mediated by a massive
graviton in Quantum Gravity, a massless graviton s-channel exchange breaks
orthogonality when considering its interference with a scalar-particle
s-channel exchange, whenever all the external states are massive. As a
consequence, we find that, in the Einstein theory, unitarity implies that
angular momentum is not conserved at quantum level in the graviton coupling to
massive matter fields. This result can be interpreted as a new anomaly,
revealing unknown aspects of the well-known van Dam - Veltman - Zakharov
discontinuity.
