ACR READ BACK ALT ASSIGNMENT THAT WAS GIVEN BUT NOT INTENDED. CTLR DID NOT CATCH THE ERROR AND DISCOVERED IT ONLY WHEN THE ACFT DSNDED BELOW ALT THAT HE EXPECTED.

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GIVEN DSCNT CLRNC TO FL180 BY ARTCC. ACKNOWLEDGED CLRNC AND READ BACK FL180. DSNDING THROUGH FL193; ATC ASKED IF WE WERE LEVEL AT FL200. WE INDICATED OUR DSCNT CLRNC TO FL180 AND HE IMMEDIATELY TOLD US TO CLB BACK TO FL200. THE CTLR WAS OVERWORKED WITH A RWY CHANGE INTO SJC (RWY 12 TO RWY 30) AND THE RESULTANT STAR CHANGES (JAWWS TO ELNIDO). WHEN THE EVENT OCCURRED; HE TOLD ACR X (US) TO TURN R 40 DEGS AS WELL. HE ALSO TOLD ACR X TO DSND IMMEDIATELY; HE REALLY MEANT TO GIVE THE CLRNC TO ACR Y INVOLVED. OBVIOUSLY THE CTLR WAS CONFUSED AND OVERWORKED; AND THE ATC TAPES WILL PROVE US RIGHT. IT REALLY DOESN'T MATTER WHO WAS RIGHT; JUST THAT THERE WAS A SIMPLE MISCOM THAT COULD HAVE PRODUCED A DANGEROUS SIT. WE DON'T KNOW IF THERE WAS A LOSS OF SEPARATION; BUT WE DO KNOW THAT WE DIDN'T GET ANY TCASII ALERTS. I REALLY WONDER WHAT THE ULTIMATE OUTCOME OF THE FAA'S NEW 'POLICY' CONCERNING READBACKS AND THE LACK OF PROTECTION THEY PROVIDE NOW. WHY EVEN BOTHER READING ANYTHING BACK? I ALSO WONDER WHAT HAPPENS WHEN THE CTLR IS AT FAULT. THE PERCEPTION OF THESE POINTS LEADS PROFESSIONAL AVIATORS TO BELIEVE THAT WE ARE ALWAYS SUSPECT AND NOT TRUSTED.