  We study the phase-space behaviour of nearby trajectories in integrable
potentials. We show that the separation of nearby orbits initially diverges
very fast, mimicking a nearly exponential behaviour, while at late times it
grows linearly. This initial exponential phase, known as Miller's instability,
is commonly found in N-body simulations, and has been attributed to short-term
(microscopic) N-body chaos. However we show here analytically that the initial
divergence is simply due to the shape of an orbit in phase-space. This result
confirms previous suspicions that this transient phenomenon is not related to
an instability in the sense of non-integrable behaviour in the dynamics of
N-body systems.
