  We show that causality is violated in the early stages of state-of-the-art
heavy-ion hydrodynamic simulations. Up to $30\%$ of the initial fluid cells
violate nonlinear causality constraints while the rest are uncertain. Only
after 2-3 fm$/c$ of evolution do we find that $\sim 50\%$ of the fluid cells
are definitely causal. Inclusion of pre-equilibrium evolution significantly
reduces the number of acausal cells, but it does not eliminate them. Our
findings suggest that relativistic causality places stringent constraints on
the available model parameter space of heavy-ion collision simulations.
