It is routinely assumed that galaxy rotation curves are equal to their circular velocity curves (modulo some corrections) such that they are good dynamical mass tracers. We analysed 33 low-mass galaxies from the APOSTLE simulation suite to explore the limits of validity of this assumption. Only 3 galaxies have rotation curves similar to their circular velocity curves; the rest are undergoing a wide variety of dynamical perturbations. We assessed how many galaxies are likely to be strongly perturbed by processes in several categories: mergers/interactions, bulk gas flows, non-spherical DM halo, warps, and IGM ram pressure. Most galaxies fall into more than one of these categories; only 5/33 are not in any of them. The sum of these effects leads to an underestimation of the low-velocity slope of the baryonic Tully-Fisher relation that is difficult to avoid, and could contribute to the observed diversity in low-mass galaxy rotation curve shapes.
