  We study the decay of a thermally excited metastable vacuum in classical
field theory using real-time numerical simulations. We find a lower decay rate
than predicted by standard thermal theory. The discrepancy is due to the
violation of thermal equilibrium during the critical bubble nucleation. It is
reduced by introducing dissipation and noise. We propose a criterion for the
system to remain in equilibrium during the nucleation process and show that it
is violated in the Hamiltonian evolution of a single field. In the case of many
fields, the fulfillment of the criterion is model-dependent.
