CLRED TO FL360; A320 FLT CREW SETS FL370 EVEN THOUGH THEY FOLLOW SOP VERIFICATION PROCEDURES.

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WE WERE IN CRUISE WBOUND AT FL340. CONVECTIVE ACTIVITY AND A LAYER OF CLOUDS JUST BELOW OUR ALT AHEAD REQUIRED A CLB. WE ASKED FOR FL360; AND WERE CLRED TO CLB. FO; PF; SELECTED FL360 ON THE ALT PRESELECT AND STATED; '36000 FT.' I LOOKED AT THE FMA AND VERIFIED '36000 FT' DISPLAYED. ALT KNOB WAS PULLED TO INITIATE CLB. THIS WAS ALL PER OUR SOP. WE RESUMED A CONVERSATION. I VERIFIED '35 FOR 36' AS WE PASSED FL350. FO RESPONDED SAME. NEXT EVENT WAS A CALL FROM CTR ABOUT OUR CONTINUED CLB PAST FL360. I LOOKED AS WE WERE PASSING FL367. ALT PRESELECT SHOWED FL370. I QUICKLY TOOK CTL AND IMMEDIATELY SELECTED FL360 AND DSNDED TO LEVEL OFF. THERE WAS NO TFC CONFLICT. I DO NOT KNOW HOW FL370 WAS SELECTED. ALL I CAN SURMISE IS THE PF TURNED THE KNOB INADVERTENTLY TO FL370 AS IT WAS PULLED TO INITIATE THE CLB. WHILE I VERIFIED THE 1000 FT TO LEVEL OFF; I MISSED RE-VERIFYING THE LEVELOFF ALT SELECTED ON THE FMA. THAT ERROR WAS OUR LAST LINE OF DEFENSE; AS THE PF MISSED THE INCORRECT LEVELOFF ALT AS WELL. CONTRIBUTING FACTOR WAS A CONVERSATION TAKING PLACE DURING THIS ALT CHANGE. ANOTHER CONTRIBUTING FACTOR WAS MY LAPTOP OPEN ON THE WORK TABLE; WHICH I LOOKED PAST TO VERIFY CHANGES. I SUGGEST THAT THE SOP BE CHANGED TO REQUIRE THAT THE ALT SELECTED IN THE PRESELECT BE SELECTED AND INITIATED (BY PULLING OR PUSHING) PRIOR TO VERIFICATION. THAT WOULD ELIMINATE AN INADVERTENT TURNING OF THE ALT PRESELECT KNOB WHILE INITIATING THE REQUIRED CHANGE (PULLING OR PUSHING). WE VERIFIED AND LOOKED AWAY PROBABLY JUST AT THE INSTANT THAT THE KNOB MUST HAVE BEEN TURNED.