  We revisit the computation of bubble wall friction during a cosmological
first-order phase transition, using an extended fluid Ansatz to solve the
linearized Boltzmann equation. A singularity is found in the fluctuations of
background species as the wall approaches the speed of sound. Using
hydrodynamics, we argue that a discontinuity across the speed of sound is
expected on general grounds, which manifests itself as the singularity in the
solution of the linearized system. We discuss this result in comparison with
alternative approaches proposed recently, which find a regular behaviour of the
friction for all velocities.
