I had a door pop open 3 times during my private pilot training. First
time, early on in the training, MY door opened on takeoff in a C-150.
The CFI took the controls and I held the door ajar. We told the tower we
needed to come back because of an open door and they cleared us ... we
did a little shorter pattern than normal, and the CFI said she noticed
more drag, but no big deal. Second time in a C-152, CFI's door popped
open just after beginning the takeoff roll. He told me to abort, I
pulled the power and started to brake as he got the door shut and told
me to go ahead (we have long runways). Third time in C-152 again, CFI's
door opened at about 500 feet. I didn't even see or hear it, but CFI was
holding onto his door with both hands. I asked, "Am I scaring you that
badly?" He laughed and said "No, door's open!" I asked if I needed to
turn back, but he just shut the door and we kept going. Now, in the
passenger briefing part of the checklist, I always ask passengers to
double-check and assure that their seats and doors are secure.