  Causality is violated in the early stages of state-of-the-art heavy-ion
hydrodynamic simulations. Such violations are present in up to 75% of the fluid
cells in the initial time and only after 2-3 fm/$c$ of evolution do we find
that 50% of the fluid cells are definitely causal. Superluminal propagation
reaches up to 15% the speed of light in some of the fluid cells. The inclusion
of pre-equilibrium evolution significantly reduces the number of acausal cells.
Our findings suggests that relativistic causality may place constraints on the
available parameter space of heavy-ion collision simulations when factored into
more thorough statistical analyses.
