On either the Arrius or Arriel engine course (can't remember which) at Turbomeca it was said that the primary overspeed protection (certification requirement) was blade shedding with the electrical overspeed system being secondary.
In the early 80's the BO 105 had a problem with freewheel slippage which manifested itself as
100% N2 with idle N1. End result was that the freewheel would re-engage on shut down destroying the driveshaft. What you saw depended on whether one or both engines were running.