  We examine the validity of the classical approximation of the waterfall phase
transition in hybrid inflation from an effective field theory (EFT) point of
view. The EFT is constructed by integrating out the waterfall field
fluctuations, up to one-loop order in the perturbative expansion. Assuming
slow-roll conditions are obeyed, right after the onset of the waterfall phase,
we find the backreaction of the waterfall field fluctuations to the evolution
of the system can be dominant. In this case the classical approximation is
completely spoiled. We derive the necessary constraint that ensures the
validity of the EFT.
