ZOB CTLR EXPERIENCED OPERROR AT 12000 FT WHEN CLBING ACFT WAS CLRED THROUGH OCCUPIED ALT.

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ACR X WAS CLBING TO 11000 FT NEBOUND FROM CLE. THESE ACFT ARE HANDED OFF FROM CLE APCH TO THE YNG SECTOR AT ZOB. YOUNGSTOWN (YNG) THEN CLBS ACFT TO REQUESTED ALT AND HAND ACFT OFF TO WINDSOR (YQG) SECTOR AT ZOB. I WAS WORKING YQG WHEN THIS INCIDENT OCCURRED. ACR X CHKED ON MY FREQ CLBING TO 11000 FT. I BACK-COORD WITH YNG TO VERIFY I HAD CTL TO CLB ACFT AND ALSO VERIFIED CLE APCH HAD RELINQUISHED CTL FOR CLB. YNG CONFIRMED I HAD CTL AND I CLBED ACR X TO 15000 FT. I HAD OTHER CTL PROBS WITH CLE ARRS AND THEN NOTICED THE CONFLICT ALERT ACTIVATE BTWN ACR X AND ACR Y. ACR Y DATA BLOCK SHOW 11000 FT CLBING TO FL230 AND HAD ABOUT 6 MI BTWN ACFT AND 200+ KTS OVERTAKE. AFTER CLRING TFC CONFLICTION OVER HIMEZ INTXN; I LOOKED BACK AT CONFLICTION AND SAW ACR X AT ABOUT 11600 FT AND ACR Y ABOUT SAME ALT AND NOW LESS THAN 5 MI. I ADVISED ACR X TO STOP CLB AND LEVEL AT NEAREST ALT. HE SAID HE WOULD STOP AT 12000 FT. I THEN SAW ACR X AT 11800 FT AND ACR Y AT 11900 FT AND ABOUT 2 MI. I INSTRUCTED ACR X TO STOP CLB AND DSND BACK TO 11000 FT. I THINK CLOSEST PROX WAS ABOUT 1.5 MI AND 100 FT. A LARGE FACTOR WAS CONFUSION BTWN YNG SECTOR AND CLE APCH CONCERNING NEW MASE AIRSPACE AND LACK OF COM BTWN FACILITIES. AS ACR X DSND; HE DSNDED BACK INTO CLE APCH WITHOUT COORD FROM MYSELF OR 'R' ASSOCIATE.