  During the Leidenfrost effect, a stable vapor film can separate a hot solid
from an evaporating liquid. Eventually, after formation and upon cooling, the
vapor layer cannot be sustained and undergoes a violent collapse evidenced by
explosive boiling. Computationally, modeling this instability involves an
interplay between hydrodynamics, thermodynamics, rapid evaporation, and
length-scales from $\mu$m to cm. Selective assumptions, made to reduce
computational costs, have limited most previous studies to steady-state
investigations. Here, we combine two-phase laminar flow, heat transfer, and
evaporation in a finite-element simulation to examine the failure of
Leidenfrost vapor layers during cooling. During periods of quiescence, the
geometry of the vapor layer agrees well with steady-state lubrication theory.
In the simulations, we report the local temperature of the solid at failure,
$T_-$, which provides a lower bound for recent experimental work using the same
geometric and material conditions. Surprisingly, we find that inertial forces,
which are typically ignored in theoretical treatments of the vapor layer, are
responsible for initiating the instability leading to failure.
