The worst thing I've found is after an annual, where the mechanic left an
engine hoist chain attached to my engine, but cowled it up! Yes, I checked
the oil and filler prior to departing, but there was no way to see the chain
through the inspection door.

Considering it is an aerobatic plane, I did not roll it inverted on the way
home, which would have been my normal practice to see if anything shook
loose. It was the last time I went to them for anything.