I had a door pop open on a Baron on take-off- the door locks weren't very beefy. The violent increase in noise was what shook me up- jumped out of my skin! And after a running changeover, a previous pilot hooked out the seat belt buckle- 6 inches or so hanging out of the right door in a Cherokee. At rotate, it suddenly started loudly banging the fuselage in the wind, and not knowing what it was I flew a circuit thinking the spar was breaking!