We show that wall slip in pasty sewage sludge occurs in the form of an abrupt motion after a small (critical) deformation of the bulk. The analysis or our different results under prolonged creep tests at different stresses, after different times of rest following preshear or with different tool surfaces, tends to show that this phenomenon likely results from the unsticking of molecules or particles from the tool surface. Then the motion results from the flow of a thin layer of liquid (possibly mixed with some solid elements) which has formed along the interface while the rest of the material (bulk) remains unsheared. With a rough tool surface the flow at low apparent shear rates is also localized in a thin layer which in that case resembles the bulk material, so that the flow rate may decrease until reaching stoppage if the applied stress is not high enough, probably because of the progressive fluid restructuration.