"Forbidden" magnon scattering (i.e. scattering in which magnons are created by increasing rather than decreasing the sample magnetization) has been observed in MnSi over the temperature range from 0.3 Tc to 2Tc in a magnetic field of 7.5 kOe. The existence of "forbidden" magnon scattering is in disagreement with theories of ferromagnetism formulated on the Random Phase Approximation. The linear behaviour of the inelastic neutron scattering intensity difference between the allowed magnon and its forbidden counterpart vs magnetization can be explained by the SKP model, which is based upon a slowly fluctuating spin density picture of itinerant ferromagnetism. Normally such behaviour is expected only for a relatively "strong" ferromagnet, but this picture seems also valid for a "weak" ferromagnet like MnSi.