One of my mates thought it would be a bit of a giggle if he sent a student out to pre-flight a PA28 which was missing the propeller. The aircraft had suffered a prop-strike a few days before, the damaged propeller had been removed and the aircraft returned to the line whilst they waited for the new propeller to arrive.
The gag almost back-fired. The student never noticed the missing propeller.
The instructor found himself running out to the aircraft at high speed to prevent the student from starting the engine. Without the load of the propeller, he could have over-reved the engine.
Sometimes it's easy to find insignificant little snags (like missing screws) than it is to find bigger important ones.