MD80 FLT CREW; DURING PUSHBACK PROCS FROM THE GATE; IS NOTIFIED THAT THE ACFT AFT STAIR IS EXTENDED. PUSHBACK PROCS ARE HALTED.

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THE GND CREW HAD FINISHED THEIR LOADING AND THE AFT STAIRS HAD BEEN RAISED APPROX 15 MINS PRIOR TO DEP. MAINT PERSONNEL HAD APPARENTLY LOWERED THE STAIRS AND REPLACED A LIGHT BULB IN THE TAIL CONE AREA. WE RECEIVED THE LOGBOOK FROM MAINT FROM THE JETWAY AND BEGAN OUR DEP PROCS. THE FLT ATTENDANTS RPTED THE CABIN SECURE AND READY FOR MOVEMENT AND ALL DOOR ANNUNCIATOR LIGHTS WERE OUT. WE COMPLETED OUR CHKLISTS AND THE TUG OPERATOR RPTED 'WALKAROUND COMPLETE; RELEASE BRAKES.' WE RECEIVED PUSHBACK CLRNC AND WERE WAITING FOR A JET TO CLR BEHIND US. AS WE WAITED FOR SEVERAL MINS THE BOARDING AGENT WHO HAD MOVED THE JETWAY AWAY FROM THE ACFT LOOKED OUT FROM BEHIND THE JETWAY TO SEE WHAT WAS DELAYING OUR PUSHBACK. HE THEN MOTIONED TO ME FROM THE JETWAY AND I OPENED MY SIDE WINDOW AND HE ASKED DID WE KNOW OUR AFT STAIRS WERE DOWN. I SAID NO. I TOLD THE TUG DRIVER TO NOT PUSH AND ASKED HIM IF HE SAW THE AFT STAIRS DOWN. HE THEN SAW THE STAIRS DOWN AND SAID THEY WERE EXTENDED. MAINT WAS CALLED AS WELL AS DISPATCH AND SUPVRS. MAINT DETERMINED THAT THE ANNUNCIATOR LIGHT FOR THE AIRSTAIRS WOULD TEST; AS IT DID DURING PREFLT; BUT WOULD NOT ILLUMINATE WITH THE STAIRS NOT UP. WE DID NOT SUSPECT ANY PROBS SINCE OUR ANNUNCIATORS INDICATED ALL DOORS WERE CLOSED. THIS COULD HAVE CAUSED SOME SERIOUS ACFT DAMAGE IF IT WERE NOT FOR THE LUCK AND OBSERVATIONS OF A BOARDING AGENT THAT RECOGNIZED A DANGEROUS SITUATION AS SOON AS HE SAW IT. SUPPLEMENTAL INFO FROM ACN 688073: WE CALLED MAINT AND FOUND OUT WE HAD AN INOP AFT TAIL DOOR OPEN ANNUNCIATOR LIGHT AND DURING OUR WAIT FOR MAINT TO GIVE IT AN MEL.