No, a plane cannot legally take off (or land) with a passenger in the lavatory. FAA and international aviation regulations in 2026 require all passengers to be "seated and belted" in an approved seat during taxi, takeoff, and landing for safety reasons. If a passenger is in the bathroom when the plane reaches the runway, the flight attendants will notify the cockpit, and the pilot will hold the aircraft until the passenger is safely back in their seat. This is not just a formality; in the event of a rejected takeoff or turbulence, a person in the bathroom is at extreme risk of injury as there are no restraints. In 2026, flight attendants are trained to "clear" the lavatories and lock them from the outside before the final safety check, ensuring the cabin is fully secured for the most critical phases of flight.