We reformulate quantum particle creation in time dependent classical backgrounds using a complete orthonormal family of localized wave packets. The formalism is then applied to analyze the reheating process after inflation during which almost all matter populating the universe in the subsequent radiation era is produced by the decay of the inflaton field. We calculate the deviations in the number and the energy densities of particles produced in a volume of a given size and find that there exists (in general tiny) spatial variations on Hubble length scales over which local interactions are incapable of restoring homogeneity. This signals the destruction of perfect homogeneity attained after inflation due to the quantum nature of particle production process in reheating.