Never managed to jack stall the Puma but it certainly let you know if you pulled back on the cyclic too hard. The sudden onset of airframe vibration was very noticeable and the collective pitch gauge needle oscillated rapidly. The recovery was instant if the severity of the manoeuvre was reduced, either by cyclic or collective.
Recovery from 90 degrees nose up was best done using yaw, rather than nose down pitch. If you pushed over too sharply in that airframe it was possible to cavitate both hydraulic pumps because the oil reservoirs were unpressurised. Only did that once!
(Just like to add that I only found these things during properly authorised display flying and not during normal operations. The display was later discontinued after two subsequent pilots found out the hard way that you can only stress the Puma airframe so far before it can suddenly go pear shaped - thankfully in both cases without the very worst happening and the damage not actually occurring to the main rotor).