I was in a DC3 when the forward freight door unlatched itself on climb-out and stayed three or four inches open. The captain looked back when the door warning light came on but we gave him the thumbs-up and I managed to pull it shut with the other engineer holding my jacket. It took a good tug but nothing too demanding. After that we had a bungee cord to hold the handle in place until the worn latch aperture doublers could be replaced.
An experienced PPL holder once told me it was possible to yaw a Cessna single into a turn by opening one of the cabin doors. slightly.