  We show how an excess in the diphoton channel can be the effect of neither a
resonance nor an end-point in a cascade decay, but rather of a threshold for
virtual production of a pair of extra quarks, each with half of peak invariant
mass, onsetting in both the $gg$-initiated production and the
$\gamma\gamma$-induced decay of an off-shell $Z$ boson. For our analysis we
consider as paradigmatic example the 750 GeV excess previously seen at the end
of 2015 with the Run 2 data of the LHC but not confirmed with 2016 data.
