Lik people say, it would depends on the door. Obviously, on some airplanes, doors will open easily. Like planes used for skydiving. (that is, if then even have a door or keep a door closed).

I've never been on an airliner where the door opened up in flight. But I have on small single engine airplanes. No big deal, really.

Just keep the seatebelt on, like they tell you to.