ZKC CTLR CLBS AN MD80 FROM FL310 TO FL330. CTLR OBSERVES ACFT'S MODE C CLB CRESTING AT FL339; ISSUING CONFLICTING TFC AT FL350. PLT CALLS FACILITY STATING HE DID NOT REACH CONFLICT ALERT POSTED ALT. NO PLTDEV FILED.

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ACR X; AN MD80; WAS A DEP OFF OF STL ENRTE TO SAT. I CLBED THE ACFT TO FL310; WHERE THEY LEVELED FOR A FEW MINS. MODE C READOUT SHOWED FL310. I THEN CLBED THE ACFT TO FL330 AND HEARD THE PLT READ BACK FL330. I ENTERED A TEMPORARY ALT IN THE DATA BLOCK OF FL330. I PLACED A 'HALO' ON THE FL350 TFC (ACR Y) TO FACILITATE THE CLBING OF ACR X TO THEIR REQUESTED ALT OF FL350; ONCE LATERAL SEPARATION WAS ENSURED. I OBSERVED ACR X DATA BLOCK READOUT OF FL332 AND ATTRIBUTED IT TO BEING WITHIN +/-300 FT. ONCE I OBSERVED A MODE C READOUT OF FL334; I ASKED THE PLT TO VERIFY LEVEL AT FL330. HE RESPONDED 'WE THOUGHT WE WERE GIVEN FL350.' I REPLIED; 'NEGATIVE; YOUR CLRNC WAS FL330; DSND AND MAINTAIN FL330; TFC 12 O'CLOCK; 1 1/2 MI AT FL350.' HE RESPONDED 'ROGER; LEVELING AT FL330.' HIS DATA BLOCK SHOWED FL339 AT THIS TIME; THEN FL336; THEN FL331. THE TFC MGMNT SUPVR INTERVIEWED THE PLT VIA PHONE; RPTEDLY ASKED THE PLT IF HE EVER LEFT FL330; AND SUPPOSEDLY THE PLT SAID NO; THAT POSSIBLY THEY HAD A MODE C PROB. THE TFC MGMNT SUPVR DETERMINED IT WAS A NON OCCURRENCE AND NO PLTDEV WAS FILED.