A B767 FO; WHILE IN CRUISE; VIOLATED THE COCKPIT TWO PERSON RULE; BY LEAVING THE COCKPIT WITHOUT ASSIGNING A FLT ATTENDANT TO REMAIN IN HIS ABSENCE.

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I ACCIDENTALLY VIOLATED THE 2 PERSON RULE FOR THE COCKPIT WHEN AT CRUISE ALTITUDE. I LEFT THE COCKPIT TO USE THE LAV. AS SOON AS THE DOOR CLOSED; I REALIZED THAT I HAD SLIPPED BACK INTO OLD HABITS; AND TURNED TO OPEN IT REGRETFULLY HAD TO CALL THE CAPT TO APOLOGIZE AND ALLOW THE FA TO ACCESS IN MY ABSENCE. CALL THE FLT ATTENDANT TO ENSURE THE AREA WAS CLR; SAW THE CAPT WAS ALREADY WEARING HIS MASK; AND LEFT WITHOUT THINKING. I THINK IT WAS A CASE OF BEING LULLED INTO COMPLACENCY AND FALLING INTO OLD HABITS. A BEAUTIFUL MORNING; LAST LEG OF THE TRIP; AND GOING INTO VACATION AFTERWARDS. SUPPLEMENTAL INFO FROM ACN 543272: FO INDICATED NEED TO USE RESTROOM; I TOLD HIM I HAD THE ACFT AND PUT MY MASK ON. I NOTICED HE CALLED FLT ATTENDANT ON INTERPHONE; AND ASSUMED HE WAS COORDINATING COCKPIT ENTRY TO SATISFY TWO PERSON RULE. I HEARD HIM LEAVE; BUT DID NOT HEAR ANYONE ENTER THE COCKPIT BEHIND HIM. WHEN I TURNED AROUND; I FOUND THAT HE HAD LOCKED THE FLT ATTENDANT OUT OF THE COCKPIT. IT WAS A SIMPLE MISTAKE; AND WE IMMEDIATELY COORDINATED ACCESS FOR THE FLT ATTENDANT SO WE COULD HAVE TWO PEOPLE IN THE COCKPIT. I HAD NOT NOTICED AS HE LEFT; AS MY ATTENTION WAS DIRECTED TO FLYING THE ACFT. PERHAPS I SHOULD HAVE BEEN MORE PROACTIVE.