  We consider equations of M\"uller-Israel-Stewart type describing a
relativistic viscous fluid with bulk viscosity in four-dimensional Minkowski
space. We show that there exists a class of smooth initial data that are
localized perturbations of constant states for which the corresponding unique
solutions to the Cauchy problem break down in finite time. Specifically, we
prove that in finite time such solutions develop a singularity or become
unphysical in a sense that we make precise. We also show that in general
Riemann invariants do not exist in 1+1 dimensions for physically relevant
equations of state and viscosity coefficients. Finally, we present a more
general version of a result by Y. Guo and A.S. Tahvildar-Zadeh: we prove
large-data singularity formation results for perfect fluids under very general
assumptions on the equation of state, allowing any value for the fluid sound
speed strictly less than the speed of light.
