  Top quark loops in Higgs production via gluon fusion at large invariant final
state masses can induce important interference effects in searches for
additional Higgs bosons as predicted in, e.g., Higgs portal scenarios and the
MSSM when the heavy scalar is broad or the final state resolution is poor.
Currently, the limit setting as performed by both ATLAS and CMS is based on
injecting a heavy Higgs-like signal neglecting interference effects. In this
paper, we perform a study of such "on-shell" interference effects in $pp\to ZZ$
and find that they lead to a $\lesssim{\cal{O}}(30%)$ width scheme-dependent
modification of the signal strength. Including the continuum contributions to
obtain e.g. the full $pp\to ZZ \to 4\ell$ final state, this modification is
reduced to the 10% level in the considered intermediate mass range.
