MLG ACFT GIVEN CLRNC TO 16000 FT BUT FLC THOUGHT 12000 FT AND READ THAT BACK. ATC CTLR WAS VERY BUSY AND DIDN'T CATCH ERROR ON READBACK.

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WHILE DSNDING TO FL240 A SPD REDUCTION TO 250 KTS WAS GIVEN BY CTR. A SHORT TIME LATER WE WERE ALSO GIVEN A CLRNC TO 12000 FT. THE 'CLRED TO' ALT WAS READ BACK TO CTR; AND THE FO VERIFIED THAT I HAD SET THE CORRECT ALT IN THE ALERTER AND THEN HE VERBALLY CONFIRMED THE ALT WITH ME. AFTER APPROX 2 MINS OF LEVEL FLT AT 12000 FT AND JUST PRIOR TO 'SPANE' INTXN ON THE 'SPANE TWO' ARR; CTR CALLED TO CONFIRM OUR ALT. I RPTED LEVEL AT 12000 FT. CTR INFORMED US THAT WE HAD BEEN CLRED ONLY TO 16000 FT. I REPLIED THAT WHEN GIVEN OUR DSCNT CLRNC I HAD READ BACK 12000 FT. WE WERE THEN HANDED OFF TO APCH CTL AND THE REMAINDER OF THE FLT WAS NORMAL. LOOKING BACK; I THINK THAT THIS SIT COULD HAVE BEEN AVOIDED BY XCHKING OUR 'CLRED TO' ALT WITH PUBLISHED XING RESTR ON THE ARR. IN OUR CASE; 12000 FT WAS NOT A PUBLISHED ALT ON THE ARR. THIS CLUE SHOULD HAVE CAUSED US TO NOT DSND BELOW THE HIGHEST PUBLISHED XING RESTR UNTIL VERIFYING OUR CLRED TO ALT. A CONTRIBUTING FACTOR COULD HAVE BEEN THE HIGH WORKLOAD ON THE CTLR AND THE POTENTIAL FOR A COM BREAKDOWN.