  Over the course of the recent decades, $N$-body simulations have become a
standard tool for quantifying the gravitational perturbations that ensue in
planet-forming disks. Within the context of such simulations, massive
non-central bodies are routinely classified into "big" and "small" particles,
where big objects interact with all other objects self-consistently, while
small bodies interact with big bodies but not with each other. Importantly,
this grouping translates to an approximation scheme where the orbital evolution
of small bodies is dictated entirely by the dynamics of the big bodies,
yielding considerable computational advantages with little added cost in terms
of astrophysical accuracy. Here we point out, however, that this scheme can
also yield spurious dynamical behaviour, where even in absence of big bodies
within a simulation, indirect coupling among small bodies can lead to
excitation of the constituent "non-interacting" orbits. We demonstrate this
self-stirring by carrying out a sequence of numerical experiments, and confirm
that this effect is largely independent of the time-step or the employed
integration algorithm. Furthermore, adopting the growth of angular momentum
deficit as a proxy for dynamical excitation, we explore its dependence on time,
the cumulative mass of the system, as well as the total number of particles
present in the simulation. Finally, we examine the degree of such indirect
excitation within the context of conventional terrestrial planet formation
calculations, and conclude that although some level of caution may be
warranted, this effect plays a negligible role in driving the simulated
dynamical evolution.
