Total of 7.
Oddest was on a Sea King when the engine ran up to max but the torque dropped to zero. Aha! Obviously a free power turbine failure. No. Turned out to be air in the inlet guide vanes. They moved - because of the air [more bubbles really in the fuel that is used to control the inlet guide vanes position]- this altered the mass flow of air through the engine and importantly out the back end. This reduced the speed of the free power turbine. The good old flow control unit for the fuel detected this and poured more fuel into the engine - the gas side spins up trying to reestablish the balance - which it can never do. Net effect an engine spinning up with a compensatory loss in torque at the same time.
Took us ages to work that out.
One day I'll tell you about the apparent double drive failure