  The detection of fast quasi-periodic variability from accreting black holes
and neutron stars has been used to constrain their masses, radii, and spins. If
the observed oscillations are linear modes in the accretion disks, then bounds
can be placed on the properties of the central objects by assuming that these
modes are locally sub-Keplerian. If, on the other hand, the observed
oscillations correspond to non-linear resonances between disk modes, then the
properties of the central objects can be measured by assuming that the resonant
modes are excited at the same radial annulus in the disk. In this paper, we use
numerical simulations of vertically integrated, axisymmetric hydrodynamic
accretion disks to provide examples of situations in which the assumptions
implicit in both methods are not satisfied. We then discuss our results for the
robustness of the mass and spin measurements of compact objects from
variability studies.
