  The effect of threshold singularities induced by unstable particles on
two-loop observables is investigated and it is shown how to cure them working
in the complex-mass scheme. The impact on radiative corrections around
thresholds is thoroughly analyzed and shown to be relevant for two selected LHC
and ILC applications: Higgs production via gluon fusion and decay into two
photons at two loops in the Standard Model. Concerning Higgs production, it is
essential to understand possible sources of large corrections in addition to
the well-known QCD effects. It is shown that NLO electroweak corrections can
incongruently reach a 10 % level around the WW vector-boson threshold without a
complete implementation of the complex-mass scheme in the two-loop calculation.
