ALTDEV ALT OVERSHOT BY AN MD80 FLT CREW WHEN THE ALT ALERTER IS KNOCKED OFF THE PRESET XING ALT NW OF SFO; CA.

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LOCATION: DUXBY INTXN; 6 NM NW OF SFO. CLRNC WAS GIVEN TO CROSS DUXBY INTXN AT 12000 FT ON THE GOLDEN GATE 4 ARR TO SJC. SOMEHOW ON DSCNT; THE ALT SELECT KNOB WAS HIT AND THE ARMING ALT WAS RESET TO 10000 FT. I MISSED THAT IN THE DSCNT. UPON GOING THROUGH 11200 FT; THE CTLR ASKED US WHAT ALT HAD WE BEEN CLRED TO. I RESPONDED WITH 10000 FT. HE ASKED IF WE COULD LEVEL AT 11000 FT; AND WE DID. I ALSO ASKED IF WE HAD CREATED A CONFLICT. THE CTLR RESPONDED WITH A DEFINITE 'NO.' HE HANDED US TO BAY APCH AND WE CONTINUED THE FLT WITH NO FURTHER INCIDENCE. UPON FURTHER STUDY OF THE ARR PLATE; INDEED YOU ARE SUPPOSED TO CROSS DUXBY INTXN AT 12000 FT. THE FO RECALLS ME SETTING AND ARMING THE ALT AND HIM CONFIRMING IT. THE SHOCKER IS THAT WE BOTH MISSED IT SOMEHOW AFTER HE HIT THE ALT ARM WHEEL AND SET 10000 FT BY ACCIDENT; ON THE DSCNT AND THEN NEVER BOTHERED TO LOOK AT THE ALT ARM WINDOW AGAIN. GRANTED; WE WERE BOTH TIRED FROM THE EARLY SIGN-IN AT XA40 AND WE BOTH HAD A TERRIBLE NIGHT'S SLEEP THE NIGHT PRIOR. HOPEFULLY NO MATTER HOW TIRED I AM; I'LL KEEP CHKING AND DOUBLECHKING THINGS ON DSCNT.