MD80 CREW INBOUND TO DFW. FO WAS PF AND ON REACHING 5000 FT BELIEVED HAD BEEN CLRED TO 4000 FT AND CONTINUED TO DSND.

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LOCATION: NE ARR FOR DFW; THE BONHAM 3 ARR. WE WERE ON THE ARR; APPROX 25 DME SW OF BHM ON THE OUTBOUND RADIAL; 5000 FT; 250 KIAS. APCH GAVE US A HDG CHANGE; AIRSPD CHANGE; AND WHAT I THOUGHT WAS AN ALT CHANGE TO 4000 FT. I; THE FO AND PF; MADE THE HDG CHANGE; STARTED TO SLOW; AND BEGAN A SHALLOW DSCNT FROM 5000 FT TO 4000 FT. SHORTLY AFTER THE RADIO CALL; I NOTICED THE ALT WINDOW STILL READ 5000 FT. I ASKED THE CAPT TO SET 4000 FT IN THE WINDOW AND THAT'S WHEN HE TOLD ME THAT WE WERE NOT CLRED LOWER. I LOOKED AT THE ALTIMETER; IT READ 4800 FT. I STOPPED THE DSCNT AND CLBED BACK TO 5000 FT. I NEVER SAW LOWER THAN 4700 FT. DURING THE TRANSITION FROM 4700 FT BACK TO 5000 FT; APCH CALLED US AND TOLD US TO CLB TO 5000 FT AND WE RESPONDED THAT WE WERE CLBING BACK TO 5000 FT. NOTHING MORE WAS SAID TO US BY APCH. THE REST OF THE ARR WAS UNEVENTFUL. FROM PROB RECOGNITION TO REGAINING 5000 FT WAS APPROX 20 SECONDS. AFTER LNDG; I CALLED TRACON; ASKED IF THERE WERE ANY PROBS AND HE TOLD ME 'NO HARM; NO FOUL; NO PAPERWORK.' LESSON LEARNED: NEVER BEGIN A CLB OR DSCNT UNTIL IT IS SET IN THE WINDOW AND IS VERIFIED BY BOTH PLTS. SUPPLEMENTAL INFO FROM ACN 418170: FO WAS FLYING THE ACFT MANUALLY. MY ATTN WAS NOT ON THE ALTIMETERS WHEN THE ALT WARNING HORN SOUNDED.