  The effect of the existence of tails on the propagation of scalar waves in
curved space-time is considered via an analysis of flux integrals of the
energy-stress-momentum tensor of the waves. The geometric optics approximation
is formulated in terms of such flux integrals, and three examples are
investigated in detail in order to determine the possible effects of wave
tails. The approximation is valid for waves in Minkowski space-time (tail-free)
and waves in Schwarzschild space-time (weak tails) but it is shown how the
approximation can break down in a cosmological scenario due to destructive
interference by strong tails. In this last situation, the waves do not radiate.
