  Both observations and direct numerical simulations are discordant with
predictions of conventional stellar evolution codes for the latest stages of a
massive star's life prior to core collapse. We suggest that the problem lies in
the treatment of turbulent convection in these codes, which ignores finite
amplitude fluctuations in velocity and temperature, and their nonlinear
interaction with nuclear burning. The hydrodynamic instabilities that may arise
prompt us to discuss a number of far-reaching implications for the fates of
massive stars. In particular, we explore connections to enhanced presupernova
mass loss, unsteady nuclear burning and consequent eruptions, swelling of the
stellar radius that may trigger violent interactions with a companion star, and
potential modifications to the core structure that could dramatically impact
calculations of the core-collapse mechanism itself. These modifications may be
of fundamental importance to the interpretation of measured isotopic anomalies
in meteorites, changing the predictions of both mixing and detailed
nucleosynthesis, and of young core-collapse supernova remnants. They may also
make possible the development of an early warning system for the detection of
impending core collapse.
