From the movie "The Aviator":
The cause was an oil leak.
I've read of more occasions of propellor governors that went cuckoo in flight due to oil leakage or contamination by metallic particles due to engine wear or damage. Currently we have governor problems on multiple planes but only due to direct combat damage. Imagine a chance where props suddenly feather or like in this clip, reverse pitch (for the types that were able). On one of the occasions there was a prop that went to 100% pitch, overspeeding the engine, trashing it and at one point suddenly locked completely and the sudden forces applied on the gearbox resulted in rupture of the propellor shaft. Imagine damaged engine mounts so the whole engine-prop unit shears off.
A good warning could be a propellor that starts to hunt, less response to pilot input; suddenly increasing or lowering RPM.
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