We discuss non-factorisable contributions to two processes, $t$-channel single top production and Higgs production in vector boson fusion.
Non-factorisable contributions are often neglected since they enter for the first time at next-to-next-leading order in perturbative calculations in the strong coupling constant and are colour-suppressed.
However, due to a peculiar enhancement in the double-virtual contribution, these corrections can reach percent level in kinematic distributions, thus making them comparable in magnitude to predictions in the factorisable approximation.
We present phenomenological results for both processes in proton-proton collisions at 13 TeV.