Perhaps one could subtly trim, or add weight to a propeller blade in a way that wouldn't be easily seen by eye.
The original Hawker Hurricane propeller was made of wood (perhaps later ones were as well I don't know), and tools such as wood drills and fillers existed and were available to good quality.
So let's say you get access to the hanger, or a spare prop. You drill a hole in a blade, insert a lead rod, and seal with some nice filler, then tint and coat it, so it doesn't show to the eye. Its not something you really look for or test anyway. Or you shave some thickness away, and coat it.
Then if you did this at home, you sneak in and swap propeller blades either at night or by faking being authorised to be there.
Now let's think what happens if you do this. The prop behaves perfectly on the ground. It spins when pushed, works just fine. The engine starts up and its probably still just fine. A bit noisy maybe, not a problem.
But at takeoff and flying speed, its now no longer balanced. Every turn it makes, its off balance weightwise and shaking the prop shaft and engine, its mounts, and putting great mechanical strain on it - and think how fast those props turned in heavy use. The propeller pivot is wood too, and not that robust against such excessive vibration and strains either - and has a hole in one blade which isn't great either. The pilot won't hear anything, because headset and cockpit, and will at most feel the plane is shaking or a bit odd. I'm not sure what they'd notice.
After a little while of high speed rotating, dependent on how off balance it became, the mounts or the prop itself will suffer from fatigue, loosened fixings, or unexpected strain, and fail midair during some climb or dive, or perhaps even straight flight.
(The Americans and Israelis got the Iranian uranium centrifuges the same kind of way, with Stuxnet, so "making it shake itself until it breaks" is a proven time delayed sabotage technique, in a way.)
The one place this won't happen, is during pre flight testing......