Some things I have found so far during preflights (PA-38):-
-U/S Landing Light
-Loose starboard strobe light
-Grit in fuel
-Small dent in metalwork just below the windshield on port side... caused by the door being snatched from your grasp on a windy day... oops
Also had a funny one when I tried straining the fuel bowl and no fuel came out.
Me: "Er, there's no fuel coming out of the drain."
Instructor: "That's because the fuel selector is off."
D'oh!