  We study the contour dependence of the out-of-time-ordered correlation
function (OTOC) both in weakly coupled field theory and in the
Sachdev-Ye-Kitaev (SYK) model. We show that its value, including its Lyapunov
spectrum, depends sensitively on the shape of the complex time contour in
generic weakly coupled field theories. For gapless theories with no thermal
mass, such as SYK, the Lyapunov spectrum turns out to be an exception; their
Lyapunov spectra do not exhibit contour dependence, though the full OTOCs do.
Our result puts into question which of the Lyapunov exponents computed from the
exponential growth of the OTOC reflects the actual physical dynamics of the
system. We argue that, in a weakly coupled $\Phi^4$ theory, a kinetic theory
argument indicates that the symmetric configuration of the time contour, namely
the one for which the bound on chaos has been proven, has a proper
interpretation in terms of dynamical chaos. Finally, we point out that a
relation between these OTOCs and a quantity which may be measured
experimentally --- the Loschmidt echo --- also suggests a symmetric contour
configuration, with the subtlety that the inverse periodicity in Euclidean time
is half the physical temperature. In this interpretation the chaos bound reads
$\lambda \leq \frac{2\pi}{\beta}= \pi T_{\text{physical}}$.
