A B737-500 FLT ATTENDANT PREVENTED A CATERER FROM OPENING AN ARMED 2L DOOR AFTER THE DISARMED 2R DOOR WAS NOT USED BECAUSE IT WAS EXTERNALLY BLOCKED.

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OUR INBOUND FLT TO BOSTON WAS LATE DUE TO WX AND THE CUSTOMER SVC REPRESENTATIVE WAS EAGER TO GET OUR FLT OUT. PAX WERE BOARDED BUT WE WERE STILL HAVING THE FIRST CLASS GALLEY BOARDED WITH THE AFT GALLEY STILL NEEDING TO BE DONE. THE CUSTOMER SVC REPRESENTATIVE CLOSED THE DOOR AND PULLED THE JETBRIDGE; SO THE PURSER CALLED TO THE BACK; TOLD MY FLYING PARTNER TO ARM DOOR 2L BECAUSE CATERING WOULD BE COMING TO 2R. #3 FLT ATTENDANT CALLED BACK THE PURSER TO CONFIRM WHICH DOOR. SHE WAS AGAIN TOLD 2L. AS FLT ATTENDANT #2; THAT WAS MY DOOR. I WAS IN THE AISLE BRIEFING EXIT ROWS SO XXXX ARMED MY DOOR AND INFORMED ME WHEN I CAME TO THE BACK OF THE PLANE. ALTHOUGH I WAS NOT COMFORTABLE WITH HAVING AN ARMED DOOR WHEN CATERING WAS COMING; I LEFT MY DOOR ARMED BECAUSE FLT ATTENDANT #3 HAD CONFIRMED TWICE WHICH DOOR TO ARM. AS PER FAR 121.310; I STAYED AT DOOR 2R (THE OPENED DOOR) WITH MY BACK TO DOOR 2L. I HEARD DOOR 2L OPEN AND TURNED AROUND TO SEE THE HANDLE COMPLETELY UP AND THE CANVAS ATTACHED TO THE GIRT BAR BEING PULLED. I SCREAMED NO AND LUNGED FOR THE DOOR AND CLOSED IT BEFORE THE SLIDE DEPLOYED. A DEICING TRUCK HAD BLOCKED THE CATERER FROM USING DOOR 2R SO HE HAD GONE AROUND TO DOOR 2L. IT WAS DARK AND SNOWING AND WE DIDN'T KNOW. IT IS MY STRONG FEELING THAT A JETBRIDGE SHOULD NOT BE PULLED UNTIL ALL REASONS FOR THE DOORS TO BE OPENED FROM THE OUTSIDE ARE GONE. WE WEREN'T GOING ANYWHERE SINCE WE NEEDED CATERING. I FEEL IT WAS DANGEROUS AND UNNEEDED TO PULL THE JETBRIDGE. THAT CATERER STILL DOESN'T KNOW HE CAME SO CLOSE TO BEING HURT OR KILLED THAT DAY; BUT I KNOW. I ALSO KNOW I COULD HAVE BEEN HURT IF I HAD GRABBED THE DOOR AND THE SLIDE HAD DEPLOYED. I FEEL THAT THERE IS A HUGE CHANCE OF THIS HAPPENING AGAIN AND THERE IS A NEED TO LOOK AT THIS INCIDENT.