ABC News lists 6 Airline Travel Myths, including whether opening a door midflight can be done (it can’t):
There are a couple of reasons for this, but the big one is cabin pressure which, in effect, seals the doors shut. Plus many aircraft doors are “plug-type” in design meaning the doors are bigger than the opening unless they are rotated. Again, though, once cabins begin to pressurize, which occurs as the plane begins to taxi, well – forget it. Those doors are shut.
Good to know.