I have heard of a fatal accident on the toulouse final assembly line last year, when two mechanics were still working inside an aircraft while others performed the pressurisation test. They managed to open a door with the fuselage already well pressurized and were blown out of the door onto the hard hangar floor.
For the very same reason a warning light in the door window shows ground staff that there is some pressure remaining, so they don´t open it from the outside and are blown back down the stairs.
So it is possible to open the doors with some cabin pressure, but I don´t know if it is still possible when fully pressurized. There is some component of inward movement for the plug type doors, but this is small (just a few millimeters) and the lever is big. Maybe one of these guys that pull trucks with their teeth can open such door at cruising altitude.
The rules do not require that it is impossible to open plug type doors at full pressure, they just require that the pressure loads push them towards the closed position.