FLC DSNDED THROUGH ASSIGNED FL250 TO FL247 BEFORE CTLR ALERTED CREW THEIR ASSIGNED ALT WAS FL250.

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THE FLT WAS FROM OXR - LAS AT FL290; THE CREW WAS ISSUED SEVERAL DSCNT CLRNCS. FIRST TO FL260 AND THEN TO FL240. MYSELF AND THE COPLT CONFIRMED FL240 IN THE ALT SELECT WINDOW. AT 24700 FT THE ATC CTLR ASKED US TO VERIFY OUR ALT. THE COPLT RESPONDED WITH '24700 FT.' THE CTLR RESPONDED WITH A QUESTION; 'WHAT WAS OUR ASSIGNED ALT.' THE COPLT RESPONDED WITH FL240. THE CTLR SAID WE WERE ASSIGNED FL250. I IMMEDIATELY CLBED TO FL250. WE WERE ADVISED TO CALL ZLA ON THE TELEPHONE UPON REACHING OUR DEST. CONTRIBUTING FACTORS: 1) INCREASED CTLR WORKLOAD DUE TO SEVERAL IFR ACFT AND VFR ACFT REQUESTING VFR FLT FOLLOWING; 2) FASTER ACFT (AIRLINER) ON SIMILAR ROUTING WAS BEING VECTORED UNDERNEATH OUR ACFT; 3) DUE TO AIRSPACE RESTRS THE CTLR WAS UNDER PRESSURE TO START OUR DSCNT; 4) PLT WORKLOAD INCREASED DUE TO DSCNT PHASE; CHANGE OF COURSE; AND SEARCHING FOR TFC (OUR ACFT WAS BEING STEPPED DOWN DUE TO OVERTAKING TFC); AND 5) SEVERAL ALT CLRNCS ISSUED TO SEVERAL DIFFERENT ACFT WITHIN A SHORT TIME. QUALITY OF HUMAN PERFORMANCE: 1) PLTS ARE FAMILIAR WITH RTE DUE TO NUMEROUS FLTS TO SAME DEST (POSSIBLE COMPLACENCY); AND 2) STANDARD XING RESTR (JOKUR AT FL240) NOT ISSUED (HABITS).