A discussion is given concerning the effects of viscosity and large disturbance amplitude on the instability occurring at a slip interface between two parts of an incompressible fluid. In particular it is shown how an analysis by Carrier can be applied to explain the train of small vortices generated at a shearing edge, and comparisons with experimental results are given for a number of examples. In addition, the numerical method of Fromm has been applied to obtain complete solutions of the slip instability problem, and numerous results are presented to show additional comparisons with the linearized theory for viscous fluids, and to show in detail the structure of the late-stage nonlinear flow in which the vortex sheet may tend to ``roll up.''