"Jay Honeck" writes:

...refused to fly with someone you felt was not entirely safe? (With that
person acting as PIC, not as a passenger in your own plane.)

A couple of times. It doesn't happen often 'cause remembering mommy's
advice I usually don't consider flights with strangers.

The last time I turned down a flight was when I was looking for an
airplane to buy. In that process one does encounter questionable
planes and pilots and they usually are paired with each other. So
when I politely refused one owner's eager offer of a flight it had as
much to do with the plane as the pilot.

The other time was years ago when I was in a flying club. There, you
get to know the club members and based on their behavior on the ground
you can extrapolate to their behavior in the air.