FO OF AN AIRBUS A320; OVERSHOT THE SID DEP XING ALT DUE TO SETTING THE FMC MODE SELECTOR IN ERROR.

Narrative

COPLT WAS PF ON LOUPE 9 DEP SID TO CROSS SJC VOR AT 12000 FT WITH ALT CLRNC TO FL230. COPLT WAS USING AUTOPLT AND MANAGED CLB ON FMS MODE WITH THE PRE-PROGRAMMED RESTR OF 12000 FT AT SJC VOR. THE ACFT HOWEVER CONTINUED TO CLB HIGHER THAN 12000 FT JUST PRIOR TO SJC VOR AND COPLT STOPPED CLB BY TURNING OFF AUTOPLT AND HAND-FLYING THE ACFT BACK TO 12000 FT. MAX ALT REACHED ON CAPT ALTIMETER WAS 12400 FT. SAME ON FO ALTIMETER. ATC CTLR NOTICED THE ALTDEV AND CLRED US TO 14000 FT. WE INITIATED CLB TO 14000 FT BUT HAD ALREADY RETURNED TO 12000 FT WHEN CLB WAS INITIATED. CAPT (MYSELF) WAS DISTRACTED SLIGHTLY FROM MONITORING INSTS BY CLB CHKLIST AND FREQ CHANGE. ON ANALYSIS AND DISCUSSION WITH COPLT AS TO WHY ACFT AUTOPLT DID NOT LEVELOFF AT 12000 FT WE CONCLUDED THAT WHEN COPLT SET IN AN ALT CHANGE ON FMP FROM 12000 FT TO FL230 HE INADVERTENTLY SELECTED OPEN CLB BY PULLING THE ALT CHANGE BUTTON INSTEAD OF JUST SELECTING THE NEW ALT IN THE WINDOW. THIS WOULD HAVE DEACTIVATED THE ALT CONSTRAINT. UNFORTUNATELY NEITHER PLT CAUGHT THE MISTAKE DUE TO THE MANY TASKS THAT WE WERE ACCOMPLISHING IN A SHORT TIME FRAME.