  We re-examine the problem of the "Loschmidt echo", which measures the
sensitivity to perturbation of quantum chaotic dynamics. The overlap squared
$M(t)$ of two wave packets evolving under slightly different Hamiltonians is
shown to have the double-exponential initial decay $\propto \exp(-{\rm
constant}\times e^{2\lambda_0 t})$ in the main part of phase space. The
coefficient $\lambda_0$ is the self-averaging Lyapunov exponent. The average
decay $\bar{M}\propto e^{-\lambda_1 t}$ is single exponential with a different
coefficient $\lambda_1$. The volume of phase space that contributes to
$\bar{M}$ vanishes in the classical limit $\hbar\to 0$ for times less than the
Ehrenfest time $\tau_E=\frac{1}{2}\lambda_0^{-1}|\ln \hbar|$. It is only after
the Ehrenfest time that the average decay is representative for a typical
initial condition.
