Was giving a guy his first twin engine lesson afew years back, his Father who was an Aircraft Engineer came back seat for a ride. He spent most of the hour yelling Instructions at his son, and telling him a much better way to do things..........
After the flight he showed his son a 2 hour preflight inspection, covering everything I had missed !
After telling Dad he was not welcome for the next flight he let go with how little I new, I happily gave him the phone numbers for my boss, and the CAA, to voice his concerns.
I never heard back from either, but Son did very well when Dad did not fly back seat again !