ZID CTLR EXPERIENCED OPERROR AT FL240 WHEN ISSUED ALT DIFFERED FROM ALT ENTERED INTO THE DATA BLOCK.

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ACR X WAS BEING SPACED BEHIND ACR Y. I MEANT TO ISSUE FL250. I ISSUED FL240 BY MISTAKE. I ENTERED FL250 INTO THE DATA BLOCK. THIS WK OUR PLUG-IN TIME IS WAY UP BECAUSE OF A 'FULL COURT PRESS' AND A CBI CALLED 'NAT PRO.' THE OPERATIONAL MGRS IN THE BUILDING ARE NOT ALLOWING FIRST LINE SUPVRS TO COMBINE AND 1 MAN SECTORS WHEN IT IS SLOW; SO PLUG-IN TIME IS WAY UP. SUPPLEMENTAL INFO FROM ACN 674006: DURING DSCNT ON CINCE 4 ARR; WE WERE CLRED BY ZID (FREQ 132.82) TO DSND FRO FL320 TO FL240. WE BOTH ACKNOWLEDGED THE CLRNC BY READING BACK CLRNC; PF PUTTING NEW ALT (FL240) IN THE MCP AND PNF POINTING TO THE NEW ALT IN THE MCP ALT WINDOW. PASSING THROUGH FL250; WE RECEIVED A TCAS ALERT AND VISUALLY ACQUIRED TFC AT 12 O'CLOCK POS PASSING R TO L AT FL240 AND APPROX 6-8 NM. WE SLOWED OUR DSCNT RATE AND TRIED TO CALL CTR ABOUT TFC; THEN HE ASKED US IF WE WERE LEVELING AT FL250. WE ACKNOWLEDGED; 'NO; WE ARE DSNDING TO FL240.' HE THOUGHT WE HAD BEEN GIVEN FL250 AND ASKED US TO CLB BACK TO FL250. WE BEGAN OUR CLB WHEN HE (ZID) RECLRED US TO FL240 AND GAVE US A FREQ CHANGE. BOTTOM LINE: WE EITHER HEARD THE WRONG ALT CLRNC OR PUT THE WRONG ALT MISTAKENLY IN THE MCP AND DIDN'T CATCH OUR MISTAKE OR ZID GAVE US A DIFFERENT ALT CLRNC THAN THEY INTENDED TO.