  The B\"{o}deker-Moore thermal friction is usually used to determine whether
or not a bubble wall can run away. However, the friction on the wall is not
necessarily a monotonous function of the wall velocity and could have a maximum
before it reaches the B\"{o}deker-Moore limit. In this paper, we compare the
maximal hydrodynamic obstruction, a frictional force that exists in local
thermal equilibrium, and the B\"{o}deker-Moore thermal friction. We study the
former in a fully analytical way, clarifying its physical origin and providing
a simple expression for its corresponding critical phase transition strength
above which the driving force cannot be balanced out by the maximal
hydrodynamic obstruction. We find that for large parameter space, the maximal
hydrodynamic obstruction is larger than the B\"{o}deker-Moore thermal friction,
indicating that the conventional criterion for the runaway behavior of the
bubble wall may have to be modified. We also explain how to apply efficiently
the modified criterion to particle physics models and discuss possible
limitations of the analysis carried out in this paper.
