A LIFEGUARD ATP PLT DSNDS BELOW THE ALT THAT ATC THOUGHT THEY HAD ISSUED; WITH ATC ADVISING PLT OF DEV. ATC THEN RECLRED FLT TO THE SAME ALT AS FIRST CLRED TO 2000 FT ON FREQ WITH ZZZ; US.

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I WAS FLYING ON AN IFR FLT PLAN FROM XXX TO YYY. FLT CONDITIONS WERE VMC. LEVEL AT 14000 FT MSL; I WAS GIVEN THE FOLLOWING CLRNC: '...CROSS INTXN AT AND MAINTAIN 12000 FT....' I READ BACK THE CLRNC AND DIALED THE ALT IN THE ALT ALERTER BOX. APPROX 1 MIN LATER; I MADE THE FOLLOWING XMISSION TO CTR: 'CTR; (MY CALL SIGN) IS LEAVING ONE-FOUR THOUSAND FOR ONE-TWO; TWELVE THOUSAND.' ZSE RESPONDED WITH 'ROGER.' AS I WAS PASSING 12600 FT; ZSE MADE A CALL TO ME STATING MY CLRED ALT WAS 'ONE-THREE THOUSAND;' AND HE WAS SHOWING ME APCHING 'ONE-TWO THOUSAND FIVE HUNDRED.' I STOPPED MY DSCNT AND SAID THAT I WAS SORRY; I HAD BACK 'ONE-TWO; TWELVE THOUSAND.' CTR THEN RESPONDED BACK THAT I COULD CONTINUE DSCNT TO MAINTAIN 'ONE-TWO THOUSAND.' I WAS HANDED OFF TO APCH SHORTLY THEREAFTER. I HAVE ALWAYS BEEN EXTRA VIGILANT ABOUT ALT ASSIGNMENTS AND FOLLOW APPROPRIATE SOP'S/PROTOCOL RELIGIOUSLY. I AM NOT SURE WHERE THE CONFUSION STARTED; BUT CLRLY THERE WAS A MISCOM. MY ONLY THOUGHT IS THAT IF I; IN FACT; HAVE SOME SORT OF CULPABILITY IN THE MISCOM; THAT THIS EVENT WILL DEMONSTRATE A FEW THINGS: 1) EVEN A HIGH DEG OF AWARENESS AND PROC CANNOT PROTECT AGAINST THIS SORT OF SIT UNLESS BOTH PLT AND CTLR SHARE THE SAME DEG OF ALERTNESS AND PREVENTION AT THE SAME TIME. 2) NO MATTER HOW EXPERIENCED; EACH PLT HAS A LOT TO RE-LEARN EACH FLT. COMPLACENCY MAY NOT BE OBVIOUS; IT CAN BE SUBTLE AND NOT JUST BE IN THE COCKPIT.