CAPT OF AN MLG ACR ACFT MISTOOK ATC CLRNC OF A HDG CHANGE FOR AN ALT CHANGE RESULTING IN A DEV FROM ASSIGNED ALT AND TRACK.

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THE FLT WAS AT CRUISE ALT OF FL350 ENRTE TO CMH. WE HAD BEEN CLRED DIRECT HNN APPROX 2 MINS EARLIER. I WAS WORKING RADIOS; CAPT WAS FLYING. I WAS IN THE PROCESS OF LOCATING CMH APCH PLATES IN MY MANUAL WHEN WE WERE ASSIGNED A HDG OF 330 DEGS FOR TFC. I ACKNOWLEDGED THE HDG; SET MY HDG BUG; AND RESUMED FLIPPING THROUGH MY CHARTS. (PREVIOUS HDG FOR DIRECT HNN WAS 325 DEGS.) AT THIS TIME; LEAD FLT ATTENDANT ENTERED COCKPIT. THE NEXT THING I KNEW; I LOOKED UP FROM MY CHARTS AND SAW WE WERE AT FL340 AND DSNDING. I CHKED ALT WINDOW AND SAW IT WAS SET AT 33000 FT. I WAS COMPLETELY STARTLED BY THIS AND KNEW WE HAD NOT BEEN GIVEN LOWER ALT. I QUESTIONED CAPT. HE SAID WE HAD BEEN CLRED TO 330. I KNEW OUR TFC WAS AT FL330 AND TCASII VERIFIED THIS. I TOLD CAPT THE CLRNC WAS FOR HDG 330 DEGS NOT ALT 330. HE THEN AGREED AND CLB WAS IMMEDIATELY INITIATED BACK TO FL350. TCASII DID ISSUE A TA FOR APCH 1 SECOND JUST AS CLB WAS INITIATED. ATC WAS NOTIFIED OF DEV IMMEDIATELY. THIS EVENT SHOOK ME UP CONSIDERABLY. I DON'T REMEMBER THE ATC FACILITY BECAUSE OF THIS AND ALSO DUE TO SEVERAL FREQ CHANGES IN A SHORT TIME SPAN. I BELIEVE IT WAS IND; IF NOT IT WAS ATL. AS FOR SEPARATION; I BELIEVE THE OTHER ACFT HAD JUST BARELY BROKEN THE 10 DME ARC OF OUR TCASII DISPLAY AND VERT SEPARATION NEVER WAS LESS THAN 1000 FT. AS FOR WHAT COULD HAVE PREVENTED THIS OCCURRENCE; I'M NOT REALLY SURE. MISCOM IS A CONSTANT THREAT; ONE THAT I TRY TO AVOID WITH GREAT VIGILANCE. THE CAPT'S FAILURE TO UTILIZE PROPER ALT AWARENESS PROCS WAS PROBABLY THE BIGGEST CONTRIBUTING FACTOR.