The origin of a series of discrete peaks in the scattered ion spectra of Si scattered off a Cu(111) surface at glancing angles has been described by the authors as transient adsorption or skipping motion. Various experimental tests have been applied to the effect and subsurface channelling or other crystallographic effects have been eliminated as possible explanations [K.J. Snowdon, D.J. O'Connor and R.J. MacDonald, Phys. Rev. Lett. 61 (1988) 1760; Appl. Phys. A47 (1988) 83; Radiat. Eff. Defects Sol. 109 (1989) 25]. This leaves the trapping in the surface binding potential as the preferred model. Of the likely trapping mechanisms which could be responsible, one not seriously addressed so far is a kinetic process involving scattering from either steps on the surface or atoms experiencing extreme thermal displacements. These deflected projectiles could be trapped in a surface binding trajectory which results in an oscillatory path perpendicular to the surface.