interesting topic, especially good reading for students!
In 5 years of light aviation flying I had the following:
-found loose oil filler cap (the previous pilot had a lot of luck I guess) (PA18)
-no rpm drop during testing of carb heat (found to be the connection rod between engine and lever loose under the hood) (PA18)
-during mag check found engine rpm varying rapidly between 1000 and 1700 rpm on one magneto. (PA18)
- found one main tyre partly deflated (c152)
-also had the pax door opening on take off, since then is an important point on my preflight
-flat battery (prop manual crancking, on PA18)
-spring of tailwheel not connected to rudder (PA18)
-full fuel tanks found on a C152 with long range tanks, =overweight with a passenger!
-front window full of bugs, hard to fly VFR
-of course the occasional low oil
- I once saw a C152 taxiing with the nose gear tow bar connected.
As mechanic on the big stuff, I have to admit these planes fly with a LOT of known problems, and nearly every flight brings up some things that are not right.