  We revisit the one-loop corrections on CMB scale perturbations induced from
small scale modes in single field models which undergo a phase of ultra
slow-roll inflation. There were concerns that large loop corrections are
against the notion of the decoupling of scales and they are cancelled out once
the boundary terms are included in Hamiltonian. We highlight that the
non-linear coupling between the long and short modes and the modulation of the
short mode power spectrum by the long mode are the key physical reasons behind
the large loop corrections. In particular, in order for the modulation by the
long mode to be significant there should be a strong scale-dependent
enhancement in power spectrum of the short mode which is the hallmark of the
USR inflation. We highlight the important roles played by the would-be decaying
mode which were not taken into account properly in recent works claiming the
loop cancellation. We confirm the original conclusion that the loop corrections
are genuine and they can be dangerous for PBHs formation unless the transition
to the final attractor phase is mild.
