  Nucleation rate computations are of broad importance in particle physics and
cosmology. Perturbative calculations are often used to compute the nucleation
rate $\Gamma$, but these are incomplete. We perform nonperturbative lattice
simulations of nucleation in a scalar field theory with a tree-level barrier,
computing a final result extrapolated to the thermodynamic and continuum
limits. Although the system in question should be well-described by a complete
one-loop perturbative calculation, we find only qualitative agreement with the
full perturbative result, with a 20% discrepancy in $|\log \Gamma|$. Our result
motivates further testing of the current nucleation paradigm.
