AN ACR B737-500 FLC DSNDS BELOW 12000 FT ASSIGNED WHEN THE FO PUTS 10000 FT IN THE ALT ALERTER IN ERROR.

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WE WERE ABOUT 1/2 WAY FROM GSP TO ATL; WHILE DSNDING TO 16000 FT ON THE SINCA 3 ARR. CTR CLRED US DIRECT TO CANUK AND TO CROSS IT AT 250 KIAS AND 12000 FT. THEN; ABOUT 10 MI FROM CANUK; ATC SAID THAT 'ATL APCH JUST SLAMMED THE DOOR; CLRED DIRECT TO SINCA TO HOLD AS PUBLISHED; EFC XA30' (ABOUT 20 MINS AWAY). I ENTERED THE NEW CLRNC INTO THE FMC AND NOTICED IT WAS ONLY 10 MI AWAY. IT WAS ABOUT THAT TIME THAT THINGS GOT A LITTLE RUSHED. THE NEW CLRNC CAUSED THE AUTOPLT TO DROP OUT OF 'VERT NAV' MODE INTO A 'LEVEL CHANGE' FAIRLY STEEP DSCNT. WHILE PROGRAMMING THE FMC WITH THE HOLDING PATTERN AND CORRECTING THE DSCNT I NOTICED THAT 16000 FT WAS STILL IN THE ALT WINDOW. IN MY RUSH I NEGLECTED TO DOUBLECHK THE RESTR AND I PUT 10000 FT IN INSTEAD OF 12000 FT. I PROBABLY PUT 10000 FT IN BECAUSE HOUSTON IS MY MOST COMMON DEST AND IT HAS MAINLY 10000 FT RESTRS. THIS ALL HAPPENED ABOVE 17000 FT IN THE DSCNT AND PRIOR TO REACHING THE HOLDING PATTERN. WE CROSSED SINCA DSNDING THROUGH ABOUT 15700 FT USING A PARALLEL ENTRY. THE CAPT RPTED TO ATC THAT WE WERE ENTERING HOLDING; THE CURRENT ALT AND DSNDING TO 10000 FT (WHICH HE READ OUT OF THE ALT ALERT WINDOW). NEITHER THE CAPT NOR THE CTLR CAUGHT MY ERROR. BEFORE WE STARTED OUR INBOUND TURN; CTR CLRED US DIRECT TO CANUK AND TO CROSS IT AT 12000 FT. BY THEN WE WERE AT ABOUT 11500 FT AND QUICKLY STARTED A CLB.