The weld that hold the magnets in the magnetos fails. The mag still spins but slower than before and fire random sparks into the engine causing the engine to slow down insidiously.
If you are clever and you realise what has happened you can turn off the failing mag, or you land in a field.
OR
A con rod has stuck itself out of the side of the crankshaft.
OR
A hole in the carburretor diaphragm
OR
You have flown inverted for longer than the endurance of the inverted fuel tank
OR
You are flying a Tiger moth and pull a loop. Everyone knows the engine stops at the top because of fuel starvation but starts again when gravity mode is reselected. Except this engine is new and a bit tight and stops instead of windmilling so it won't restart (and there's no starter motor).
OR
A section of the propellor has fallen off
OR
All of the propellor has fallen off
OR
You just pulle dthe Mixture knob thinking it was the carb heat
OR
You were trying to change tanks and you turned it off instead.
.....
It goes on, just read the accident reports. Theres a joke that pilots don't parachute because who would jump out of a servicable aircraft. The quick response from the meat bombs is 'Theres no such thing as a serviceable aircraft.'
All of this leads to the conclusion that if you are going to fly make sure you are happy with the forced landing drill 'cos one day it will happen. And if the engine stops undo what you just did.
On the bright side mechanical failure does not feature too highly in accident reports and most planes are pretty reliable.