  We examine the early phases of two near-limb filament destabilization
involved in coronal mass ejections on 16 June and 27 July 2005, using
high-resolution, high-cadence observations made with the Transition Region and
Coronal Explorer (TRACE), complemented by coronagraphic observations by Mauna
Loa and the SOlar and Heliospheric Observatory (SOHO). The filaments' heights
above the solar limb in their rapid-acceleration phases are best characterized
by a height dependence h(t) ~ t^m with m near, or slightly above, 3 for both
events. Such profiles are incompatible with published results for breakout,
MHD-instability, and catastrophe models. We show numerical simulations of the
torus instability that approximate this height evolution in case a substantial
initial velocity perturbation is applied to the developing instability. We
argue that the sensitivity of magnetic instabilities to initial and boundary
conditions requires higher fidelity modeling of all proposed mechanisms if
observations of rise profiles are to be used to differentiate between them. The
observations show no significant delays between the motions of the filament and
of overlying loops: the filaments seem to move as part of the overall coronal
field until several minutes after the onset of the rapid-acceleration phase.
