A B737-300 FLC HAS AN ALTDEV FROM A CLRNC INTERP PROB DURING A PLANNED CLB THAT WAS ABOVE THEIR ACTUAL ASSIGNED ALT ON FREQ WITH ZAU; IL.

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WE HAD JUST GOTTEN AIRBORNE FROM DSM AND WERE ENRTE TO ORD ON LAST LEG OF 4. A LITTLE TIRED AND APCHING 12 HRS DUTY TIME. WE HAD WAITED OVER 30 MINS FOR OUR TKOF CLRNC DUE TO OVERHEAD STREAM TO ORD. OUR B737-500 WAS LIGHT WITH ONLY 13 PAX. INITIAL ALT CLRED TO WAS 5000 FT WITH SUBSEQUENT CLBS TO 14000 FT AND FL190. LAST CLRNC WITH FIRST CTR CTLR WAS RECEIVED AND ACKNOWLEDGED AS FL330. WE WERE DOING OVER 3500 FPM. FO CHKED IN WITH NEXT CTR OUT OF FL220 CLBING FL330. CTR ACKNOWLEDGED AND SAID HE WOULD HAVE HIGHER SHORTLY. THIS KIND OF MADE US THINK HOW HIGH THEY WOULD TAKE US SINCE WE WERE FILED FOR FL290. THE FO ASKED CTR; 'HOW HIGH ARE WE GOING? WE ARE ALREADY CLRED TO FL330.' THE CTLR CAME BACK AND SAID 'YOU WERE PROBABLY CLRED TO FL230 NOT FL330; BUT CONTINUE UP TO FL290.' I HAVE NEVER HAD SUCH A FEELING OF TERROR AND RELIEF SO CLOSE TOGETHER BEFORE. WE WERE CLBING THROUGH FL240 AS THE ABOVE EXCHANGE TOOK PLACE. THERE WAS NO LOSS OF SEPARATION OR ABRUPT MANEUVERING REQUIRED; JUST MY HEARTBEAT STOPPING FOR A SECOND. WE HAD NOT QUESTIONED FL330 INITIALLY SINCE WE BOTH 'ASSUMED' THAT WAS HOW CTLR WAS GETTING US INTO ORD BECAUSE OF INBOUND STREAM CTL. THAT; COUPLED WITH OUR FATIGUE; AND 'GET DONEITIS' WERE CONTRIBUTING FACTORS. OF COURSE; 2 CTLRS ALSO MISSED OUR FL330 READBACKS. SUPPLEMENTAL INFO FROM ACN 536659: THIS WAS MY FIRST ALTDEV IN MY FLYING CAREER. CONTRIBUTING FACTORS: READING BACK TWICE FL330 TO 2 CTLRS WITHOUT THEM CATCHING THE MISTAKE.