MD80 FLT CREW WHILE PARKED ON THE RAMP HAS THE AFT EMER SLIDE DEPLOY AT STL.

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FLT FROM MSP TO ORD DEVIATED TO STL DUE TO WX. AT STL WE WERE TOLD THAT NO GATES WERE AVAILABLE AND WE WERE PARKED ON THE JULIET PAD. BECAUSE THERE WAS NO JETBRIDGE OR MOBILE STAIRS AVAILABLE; THE EXIT DOOR EMER SLIDES REMAINED ARMED. THE AFT AIRSTAIR DOOR WAS DISARMED TO PROVIDE ACCESS TO THE ACFT; ENABLING A WALKAROUND INSPECTION AND TO RECEIVE FLT PAPERWORK. A REQUEST WAS MADE TO CATER THE ACFT WITH SOME ICE AND BEVERAGES. AFTER SITTING ON THE RAMP FOR APPROX 1 HR; THE FUEL TRUCK ARRIVED AND THE PAPERWORK FOR THE FLT WAS DELIVERED. WHILE CREW DUTIES WERE FOCUSED ON GETTING THE FLT OUT AND CONTINUED TO ORD; A CATERING TRUCK PULLED UP TO AFT GALLEY SVC DOOR. THIS WAS AN UNOBSERVED EVENT BY ALL CREW MEMBERS. THERE WERE NUMEROUS PAX STANDING AROUND IN THE CABIN AND ONE OF THEM SAW THE CATERING PERSON LOOK IN THE WINDOW AND KNOCK ON THE DOOR. BEFORE THE FLT ATTENDANT NEAR THE DOOR COULD STOP THE PAX; THE PAX OPENED THE SVC DOOR. THE CATERING INDIVIDUAL BELIEVED THAT THE PERSON AT THE DOOR MUST BE A FLT ATTENDANT AND THEN OPENED THE DOOR ENOUGH TO BLOW OPEN THE EMER SLIDE. LUCKILY THE SLIDE DEPLOYED UNDERNEATH THE TRUCK SVC PLATFORM AND NO ONE WAS INJURED. THE SLIDE WAS REPLACED BY MAINT AND FLT THE CONTINUED ON. THIS INCIDENT COULD HAVE BEEN PREVENTED HAD THE SVC DOOR SLIDES BEEN DISARMED ONCE THE AFT STAIRS WERE LOWERED. SUPPLEMENTAL INFO FROM ACN 697382: AFTER THE AIR STAIRS WERE LOWERED; HOWEVER; I SHOULD HAVE ORDERED THE OTHER DOORS DISARMED. IN THE MULTI-TASKING MAYHEM THAT ACCOMPANIES SUCH SITS; IT DID NOT OCCUR TO ME TO CHANGE THE ARMING CONFIGN OF THE DOORS.