  We calculate one-loop correction to the two-point functions of curvature
perturbation in single-field inflation generated by cubic self-interaction.
Incorporating the observed red-tilted spectrum of curvature perturbation, the
relevant one-loop correction takes a finite value and inversely proportional to
the spectral tilt. Requiring one-loop correction to be much smaller than the
tree-level contribution leads to an upper bound on primordial non-Gaussianity.
While observationally allowed region of non-Gaussian parameter space is found
to be entirely included by the region, where one-loop correction is smaller
than the tree-level contribution, an appreciably large region has one-loop
correction larger than 1% or even 10% of the latter. If future observations
conclude non-Gaussianity falls in such a region, then it would be important to
incorporate higher-order corrections to the spectrum in order to achieve
precise cosmology. In some extreme cases, where one-loop correction has a
comparable magnitude to the tree-level contribution, it might indicate
breakdown of the cosmological perturbation theory in the context of
single-field inflation.
