The experimental investigation of the hydrodynamics of a reversible pump-turbine reduced scale model in off-design operating conditions is presented. The onset and development of flow instabilities outside the “normal” operating range, in generating mode, involving runaway and “S-shape”, are described. Wall pressure measurements, in the stator, are performed with the help of miniature piezoresistive sensors synchronized with high speed flow visualizations in the vaneless gap between the impeller and the guide vanes by air bubbles injection. The detailed analysis reveals that a rotating stall arises at runaway and increases in amplitude while the discharge is decreasing, reaching its maximum value near zero discharge. The pump-turbine experiences in the vaneless gap between the impeller and the guide vanes one rotating stall cell at sub-synchronous speed.