FLC DEPARTED ASSIGNED ALT BY 800 FT MOMENTARILY.

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WE HAD DEPARTED OMA. UPON CHKING ON WITH ZMP; WE WERE CLRED TO 16000 FT AND ADVISED OF TFC PASSING AT 17000 FT. WE LEVELED AT 16000 FT ON AN ASSIGNED HDG AND IDENTED THE PASSING TFC BOTH VISUALLY AND ON THE TCASII DISPLAY. I WATCHED AS THE TFC PASSED TO OUR N. A SHORT TIME LATER THE CTLR ISSUED US A CLRNC DIRECT IOWA CITY; A FIX WHICH WASN'T IN OUR FMS. WITH THE TFC ALREADY WELL PASSED US; I WAS EXPECTING TO HEAR A CLRNC TO CLB. SO I MISTOOK THE DIRECT IOWA CITY CLRNC AS A CLB CLRNC AND COMMENCED CLBING. I REALIZED MY MISTAKE PASSING 16800 FT AND STARTED A DSCNT DOWN TO OUR ASSIGNED ALT. THE CTLR REITERATED THE FACT WE WERE ONLY ASSIGNED 16000 FT AND WE RPTED LEVEL AT 16000 FT WHICH BY THIS TIME WE WERE AGAIN. NOTHING MORE WAS STATED BY EITHER US OR THE CTLR. I THINK IT IS VERY IMPORTANT TO KEEP A CHK ON WHAT YOU EXPECT TO BE CLRED TO AND LISTEN CAREFULLY TO WHAT YOU ARE CLRED TO. THERE WERE SOME RELATED FACTORS SUCH AS GET HOMEITIS AFTER A 4-DAY TRIP NEAR THE END OF A LONG DAY WITH DELAYED TKOF. I FLEW ALL DAY WITH NOTHING MORE TO EAT THAN THE TYPICAL ARPT SNACKS. BUT THE REAL CULPRIT WAS EXPECTING TO HEAR A CLRNC AND ACTUALLY RECEIVING A COMPLETELY DIFFERENT ONE.