In perturbative QCD, the masses of the hadrons involved in high energy reactions can usually be neglected. In this talk, I will discuss one case in which this may not be a good approximation, namely production of Kaons in electron-proton collisions at low (and not so low) beam energies. In particular, I will present a recent proposal to include hadron masses in theoretical calculations, and how these Hadron Mass Corrections can explain a large discrepancy observed in measurements performed at the HERMES and COMPASS experiments.