ZOA CTLR; PROVIDING OJT; EXPERIENCED OPERROR AT FL330 WHEN A DSCNT ALT CLRNC READBACK WAS MISSED.

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I WAS INSTRUCTING A DEVELOPMENTAL CTLR AT RADAR SECTOR 34 ZOA. ACFT #1 WAS A CRJ7; DESTINED FOR SFO ON THE MODESTO STAR AT FL380. ACFT #2 WAS A C560; ORIGINALLY CLBING TO FL330 (ACFT Y WAS LEVEL AT FL330 AT TIME OF INCIDENT). THE DEVELOPMENTAL (TRAINEE) ISSUED CLRNC TO ACFT X TO DSND AND MAINTAIN FL340. THE PLT OF ACFT X READ BACK THE CLRNC. TFC (MERGING TARGET PROCS) WAS THEN ISSUED TO ACFT X AT 2 O'CLOCK POS; 15 MI; SWBOUND; CITATION FL330. ACFT X PLT ACKNOWLEDGED. TFC (MTP) WAS THEN ISSUED TO ACFT Y AT 10 O'CLOCK POS; 15 MI; WBOUND; A REGIONAL JET DSNDING TO FL340. ACFT Y PLT ACKNOWLEDGED. LATER ACFT X WAS OBSERVED DSNDING THROUGH FL340 AND AS INSTRUCTOR I STEPPED IN AND ISSUED CLRNCS TO BOTH ACFT X AND ACFT Y WHEN I FIRST NOTICED ACFT X BELOW FL340 I INITIALLY THOUGHT IT MAY HAVE BEEN A MODE C SWAP WITH THE ACFT BELOW. MY FIRST CLRNC WAS TO ACFT X TO 'MAINTAIN FL340.' WHEN THE PLT REPLIED (WE'LL GO BACK UP TO FL340) I THEN CLRED ACFT X TO TURN R HDG 300 DEGS. ACFT X ACKNOWLEDGED. I THEN CLRED ACFT Y TO TURN 30 DEGS R. I THEN STARTED TO ISSUE TFC AGAIN TO ACFT X; BUT INTERRUPTED MYSELF TO VERIFY THAT ACFT X WAS TURNING R. IT APPEARED; ON THE RADAR SCOPE THAT THE ACFT X MAY HAVE STARTED A L TURN. HE VERIFIED THAT HE WAS TURNING R. AFTER OBTAINING SEPARATION ONCE AGAIN; I GOT BOTH ACFT ON COURSE. I ASKED THE ACFT X PLT WHAT HIS PREVIOUSLY ASSIGNED ALT WAS; AND HE RESPONDED 'FL240.'