MD80 CABIN ATTENDANT FORGOT TO DISARM REAR CABIN DOOR AFTER PARKING RESULTING IN THE EMER SLIDE DEPLOYING WHEN GND SVC PERSONNEL OPENED DOOR.

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I AM AN ACR Y FLT ATTENDANT BASED IN ZZZ. ON NOV/WED/01 I WAS ON THE SECOND DAY OF A 3 DAY TRIP ON THE B737-300-400. AN MD80 WAS SUBSTITUTED FOR ONE FLT SEGMENT (DCA-PIT FLT ABC). WE THEN...ASSUMED THE 'C' FLT ATTENDANT WOULD BE IN THE FRONT WITH THE 'A' FLT ATTENDANT. THAT'S WHERE THE 'C' FLT ATTENDANT SITS ON ALL OTHER ACFT WE WORK. SINCE THE MD80 IS NOT BASED IN ZZZ; THE ONLY THING ZZZ FLT ATTENDANTS COVER IN RECURRENT TRAINING IS TAILCONE OPS. AS THE 'B' FLT ATTENDANT; I TOOK RESPONSIBILITY FOR ARMING THE 2L DOOR. I THEN TOOK MY POS AT AFT JUMP SEAT/TAILCONE EXIT. AFTER ARRIVING AT THE GATE IN PIT; I RELEASED MY SEATBELT AND STARTED TOWARD 2L DOOR. A PAX APCHED ME AND ASKED HOW TO GET TO BAGGAGE CLAIM. AFTER A COUPLE MINS OF CONVERSATION; I MUST HAVE REVERTED TO A DC9 MODE AND FORGOT TO DISARM 2L DOOR. WE GATHERED OUR BELONGINGS AND LEFT THE ACFT. SUBSEQUENTLY; UTILITY PERSONNEL CAME ABOARD AND OPENED 2L DOOR TO THROW TRASH OUT; DEPLOYING SLIDE. FORTUNATELY; NO ONE WAS HURT. I'M SURE WE ALL TRULY REGRET THIS INCIDENT. I THINK EVERYONE INVOLVED SHARES SOME RESPONSIBILITY AND CERTAINLY WE HAVE ALL LEARNED FROM IT. ONE THING I FORGOT TO ADD -- THE COCKPIT CREW WAS A SUBSTITUTE ZZZ1 CREW AND THEY DID NOT BRIEF US IN ZZZ.