  It is generically believed that the two-body scattering is suppressed by
higher-order weak couplings with respect to the two-body decay. We show that
this does not always hold when a heavy particle is produced by forbidden decay
in a thermal plasma, where the scattering shares the same order of couplings
with the decay. We find that there is a simple and close relation between the
forbidden decay and the same-order scattering. To illustrate this point, we
consider freeze-in production of heavy dark matter via a light scalar mediator.
We point out that, when the Boltzmann (quantum) statistics is used, the
forbidden decay can contribute to the dark matter relic density at 5$\%$-24$\%$
(10$\%$-$39\%$) with a weak thermal coupling, while the contribution from the
scattering channel can be several orders of magnitude larger than from the
forbidden decay if the thermal coupling is much smaller. Such a relative effect
between the scattering and the forbidden decay could also exist in other
plasma-induced processes, such as the purely thermal generation of the
right-handed neutrino dark matter, or of the lepton asymmetry in leptogenesis.
