ARTCC RADAR CTLR MISSPOKE ALT ASSIGNMENT TO A B727 CLBING OUT OF CVG AND HAD LTSS OCCUR WITH AN MD80 AT FL280 NEAR DQN. TFC VOLUME WAS HVY AND ALL 3 POS AT THE SECTOR WERE MANNED. NO ONE CAUGHT THE ERROR UNTIL THE CONFLICT ALERT ACTIVATED AND THE ACFT WERE REACTING TO THEIR TCASII RAS.

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ON THE EVENING OF AUG/XX/96; I WAS WORKING THE RADAR HDOF POS AT DAY HIGH SECTOR. I WAS TRACKING FOR ABOUT 31 MINS WHEN THE OPERATIONAL ERROR OCCURRED. ACR X; MD80; WAS WBOUND ON J80 AT FL280. ACR Y; B727; WAS CLBING OUT OF CVG OVER FFO DIRECT DJB ON COURSE. ON INITIAL CONTACT THE RADAR MAN CLBED ACR Y TO FL250 AND I UPDATED THE DATA BLOCK. WE HAD BEEN EXTREMELY BUSY FOR THE PREVIOUS 30 MINS AND WE WERE GAINING CONFIDENCE THAT WE WERE BACK COMPLETELY UNDER CTL AND MANAGEABLE. SOON THEREAFTER THE RADAR MAN CLRED ACR Y TO FL270; I WAS OFF LINE COORDINATING. THE TAPES SHOW ACR Y READ BACK FL370. AFTER COORDINATING THE R-SIDE TOLD ME HE CLBED HIM TO FL270 AND I UPDATED THE DATA BLOCK. SOON THEREAFTER ACR Y ASKED THE R-SIDE TO VERIFY HIS ASSIGNED ALT AND THE R-SIDE TOLD THE ACFT TO MAINTAIN FL280 WHICH ACR Y READ BACK; BUT IT WAS THE ALT OF THE TFC. I DIDN'T UPDATE THE DATA BLOCK FROM FL270 BECAUSE I NEVER HEARD THE CLRNC; I DIDN'T PICK UP ON A BAD READ BACK BECAUSE THERE WAS NONE. WHEN HE ISSUED FL280 THE ACR Y READ BACK FL280. THE R-SIDE NEVER TOLD ME TO UPDATE THE DATA BLOCK TO FL280 BECAUSE HE THOUGHT HE CLRED HIM TO FL270 AS DATA BLOCK SHOWED. ALSO BECAUSE OF THIS; CONFLICT ALERT NEVER ACTIVATED UNTIL ACR Y BROKE THROUGH FL273 CLBING TO FL280 AND THAT IS WHEN ADEP ACTIVATED ALSO. THE R-SIDE HAS 10 YRS EXPERIENCE AND IS WIDELY KNOWN AS A VERY FINE CTLR. IT WAS HIS FIRST DAY BACK AFTER VACATION AND HE ISSUED THE WRONG ALT AND NEITHER MYSELF OR THE D-SIDE CAUGHT IT. SUPPLEMENTAL INFO FROM ACN 343200: ACFT #1 PASSED APPROX 4 AND A HALF MI IN FRONT OF ACFT #2 AT FL280; AND TCASII MANEUVERS WERE INITIATED BY BOTH ACFT. SECTOR VOLUME AND COMPLEXITY AT THE TIME WERE VERY HIGH; POSSIBLY CONTRIBUTING TO THE ERROR.