XING RESTR NOT MET BECAUSE ALT WINDOW IN COCKPIT INADVERTENTLY SET TO WRONG ALT. THE ALT HAD BEEN SET CORRECTLY TO FL230; BUT APPARENTLY WHEN PUSHING THE KNOB TO COMMENCE DSCNT THE ALT CHANGED 1000 FT TO FL240.

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OUR FLT WAS GIVEN A CLRNC TO CROSS A FIX AT FL230. I ACKNOWLEDGED THE CLRNC CORRECTLY AND SO DID THE CAPT WHO WAS THE PF. I ALSO VERIFIED THAT THE CAPT SET FL230 IN THE ALT SELECT WINDOW. I THEN PROCEEDED TO OBTAIN THE ATIS AND PROGRAM THE FMGS. SEVERAL MINS LATER I NOTICED WE WERE XING THE FIX LEVEL AT FL240 WITH FL240 SET IN THE ALT SELECT WINDOW. THE CAPT HAD NO EXPLANATION AS TO HOW FL240 GOT SET IN THE ALT SELECT WINDOW. THE ONLY EXPLANATION I COULD COME UP WITH WAS THAT WHEN THE CAPT PUSHED THE ALT SELECT KNOB TO BEGIN THE DSCNT; HE MUST HAVE ALSO INADVERTENTLY TURNED THE KNOB THEREBY CHANGING THE ALT SELECT TO FL240. THE ALT SELECT KNOB ON THE A320 CONSISTS OF BOTH AN INNER AND OUTER KNOB. EACH CLICK OF THE INNER KNOB WILL CHANGE THE ALT SELECT WINDOW EITHER 100 OR 1000 FT DEPENDING ON THE POS OF THE OUTER KNOB. ALTHOUGH IT IS NOT COMPANY POLICY; MANY PLTS HAVE MADE IT THEIR POLICY TO ALWAYS RETURN THE OUTER KNOB TO THE 100 FT POS AFTER MAKING AN ALT SELECTION. THAT WAY IF THEY DO ACCIDENTALLY TURN THE INNER KNOB; THE AMOUNT OF ALT ERROR WILL ONLY BE 100 FT INSTEAD OF 1000 FT. ALSO; AN ERRONEOUS ALT SELECTION OF FL229 OR FL231 WOULD BE MORE OBVIOUS.