A HEARBACK READBACK SYS ERROR OCCURRED BTWN TWO ARTCC CTLRS. THE INITIAL ERROR OCCURRED WITH THE LOW ALT CTLR WHEN A C500 PLT REQUESTED CLB TO FL260. THE LOW ALT CTLR INPUT FL250 INTO THE COMPUTER; WROTE FL250 ON THE CTL DATA STRIP; BUT CLRED THE ACFT TO FL260.

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ACFT X WAS CLBING OUT OF THE LOW SECTOR AND ACFT Y WAS LEVEL AT FL260 WBOUND. TEMPORARY ALT FL250 WAS INPUT IN ACFT X'S DATA BLOCK. WHEN ACFT X CHKED IN HE SAID HE WAS OUT OF FL240 FOR FL260. I DID NOT HEAR THE FL260. EVERY INDICATION WAS THAT HE WAS CLBING TO FL250. THE DATA BLOCK SAID IT; THE STRIP SAID IT; AND THE CTLR I RELIEVED SAID IT. I THOUGHT HE WAS CLBING TO FL250. THE CTLR IN THE LOW SECTOR HAD MISTAKENLY CLBED ACFT X TO FL260. SUPPLEMENTAL INFO FROM ACN 391005: ACFT X RPTED ON AGC SECTOR; CLBED TO FL230 BY PREVIOUS CTLR. THE SECTOR WAS PREVIOUSLY COMBINED WITH BSV SECTOR. I FLASHED ACFT X DATA BLOCK AT AIR SECTOR. AIR SECTOR CTLR COMPUTER INPUT TEMPORARY ALT FL250. I CLBED ACFT X TO FL260 AND THEN SWITCHED HIM TO AIR. I COULD HAVE SWORN THAT I CLBED ACFT X TO FL250. I WOULD HAVE BET MY LIFE ON IT. I LISTENED TO THE TAPE; AND VERY CLEARLY I COULD HEAR MYSELF CLBING ACFT X TO FL260. SUPPLEMENTAL INFO FROM ACN 391135: I CHKED IN WITH ZOB; REQUESTED FL260 (FILED FL310); CLRED TO FL230. SECOND ZOB CTLR CLRED ME TO FL260. CHKED IN WITH THIRD ZOB CTLR ADVISED 'FL240 CLBING TO FL260;' HE ACKNOWLEDGED. HE LATER ASKED 'VERIFY LEVEL AT FL250.' I REPLIED 'NEGATIVE; CLBING TO FL260' (OUT OF APPROX FL256). HE TOLD ME I HAD ONLY BEEN CLRED TO FL250 'DSND IMMEDIATELY.' I DSNDED ABRUPTLY AND INFORMED HIM I HAD BEEN CLRED TO FL260; AND CHKED IN CLBING TO FL260. SPOKE TO A SUPVR AT ZOB AFTER LNDG. HE TOLD ME HE HAD REVIEWED THE TAPES; AND I WAS CLRED TO FL260 AND I DID CHK IN CLBING TO FL260.