SMT X DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACR Y. SYS ERROR.

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I WAS INSTRUCTED TO MAKE A R TURN TO 090 DEG; MAINTAIN 5000 FT. WHILE IN THE TURN I SAW THE DEST ARPT AND ASKED FOR A VISUAL APCH. THE CTLR AMENDED THE HDG TO '030 DEG; DSND TO 3000 FT.' I ACKNOWLEDGED; STOPPED MY TURN AT 030 DEG AND SIMULTANEOUSLY STARTED MY DSCNT TO 3000 FT. AT 4500 FT; THE CTLR VERY SHARPLY CALLED AND SAID; SMT X I TOLD YOU TO REMAIN AT 5000 FT. MY PAX; AN IFR PLT; WHO WAS SITTING IN THE COPLT CHAIR; AND I EXCHANGED LOOKS OF AMAZEMENT... I IMMEDIATELY CLBED BACK TO 5000 FT. I WAS GIVEN 2-3 MORE HDG CHANGES AND DSCNT CLRNCS TO A ROUTINE LNDG. DURING TAXI; GND CTL ADVISED ME TO CALL THE ONTARIO TRACON. I CALLED AND WAS ADVISED; ACCORDING TO TRACON SUPVR; I HAD STARTED A DSCNT; THOUGHT HE HAD TOLD ME TO REMAIN AT 5000 FT. I ASKED IF THEY HAD PLAYED THE TAPE. THEY HAD NOT YET HAD TIME BUT IT APPEARED I HAD DSNDED INTO THE PATH OF ACR Y AT 4000 FT; WBOUND. SINCE I HAD ANOTHER ENRTE STOP TO MAKE BEFORE RETURNING TO HOME BASE; IT WAS SUGGESTED I CALL TRACON AGAIN. I CALLED AND THE TAPE CONFIRMED THAT I HAD BEEN CLRED TO 3000 FT ALTHOUGH; AT FIRST; THE CTLR TRIED TO DENY IT. APPARENTLY; A STUDENT CTLR WHO HAD BEEN OBSERVING; CONFIRMED; I HAD BEEN CLRED TO 3000 FT; TOO. I REGRET THE CTLR MADE AN ERROR. I RESENT HIS TRYING TO BLAME 'THE PLT.' I BELIEVE MANY PLTS FEEL; UNFORTUNATELY; THE 'SYS' OFTEN 'TRIES' TO BLAME PLTS WHEN THEY (PLTS) ARE NOT AT FAULT.