I've strained out water from fuel tanks - (not caused by condensation)- a leaky seal at the filler end on one wing was permitting ingress.
Flat tailwheel tyre when I flew a taildragger with an inflatable tailwheel.
Blown fuse in R22 BII - discovered when trying to engage clutch.
Various blown bulbs - particularly Robinson landing lights.
Dent in cowling discovered immediately after release of R44 from workshop I once used - no responsibility admitted by engineering so I never went back and now warn others off. New workshop ensures my aircraft is clean and polished when I go to collect it and is serviced by people who seem to take pride in what they do.
Cockpit full of grass - thoughtful mower man had cut grass next to adjacent cockpit vent.
Aircraft covered in grass - the day after I have cleaned and polished it and the thoughtful mower man had once again struck.
Hole in tail rotor blade - Never properly resolved cause for this - maybe a stone, maybe other ground equipment. Grounded me for a while.
IMHO the best opportunity to discover problems is when you are washing the aircraft. No dent or ding will be able to hide.
So it's worth doing, it gets quicker but no less thorough with time and familiarity.
SB