  A relatively simple model problem where a single electron moves in two
relativistically-strong obliquely intersecting plane wave-packets is studied
using a number of different numerical solvers. It is shown that, in general,
even the most advanced solvers are unable to obtain converged solutions for
more than about 100 fs in contrast to the single plane-wave problem, and that
some basic metrics of the orbit show enormous sensitivity to the initial
conditions. At a bare minimum this indicates an unusual degree of
non-linearity, and may well indicate that the dynamics of this system are
chaotic.
