On 6 September 2023, control of a Boeing 738-800 was temporarily lost approaching cruise altitude. When reaching to unlock the flight deck door after a cabin crew access request, the captain operated the rudder trim switch to its full travel. Neither pilot recognised the error, and an EGPWS alert followed as left bank increased towards its maximum autopilot-engaged limit of 42°. This prompted successful upset recovery action. However, the underlying cause was recognised only after the first officer suggested checking the rudder trim position. Contrary to company procedures, prior positive identification of the intended switch had not preceded activation.