The worst - trim connected the wrong way prior to a test flight! The same a/c had the pitot/static mixed up - noticed no ASI on take-off roll.
In the last few months.....
Broken exhaust joint
Broken exhaust stud
loose spats
blocked pitot
tailspring on point of collapsing
On my own a/c I notice most of the defects when I wash/polish it. The w/round then becomes more of a confirmation of serviceability than a defect spotting exercise.