ALT DEV ALT UNDERSHOT. ALT XING RESTRICTION NOT MET.

Narrative

FO FLYING AND WE WERE CLRED TO CROSS 45 E OF CGT AT 10000 FT. AT ABOUT 50 E I SAW WE WERE PASSING THROUGH 10600 FT AND I HAD LCL ALTIMETER SET. NOT NOTICING WE STOPPED AT 10400 (LEVELED OFF); CENTER CALLED US AT 40 E TO QUESTION OUR ALT AND THEN WE DISCOVERED THE FO HAD INADVERTENTLY RESET HIS ALTIMETER BUTTON TO STD (29.92); SO HIS AUTOPLT LEVELED US OFF AT 10400 FT MSL. CENTER SAID NO CONFLICT RESULTED. WE TRIED TO FIND OUT WHY THE FO'S ALTIMETER WAS BACK AT STD AND NOT THE LCL ALTIMETER AS HAD BEEN SET DURING OUR PRELIMINARY LNDG CHKLIST. ON A SUBSEQUENT FLT; THE FO FOUND THAT HE WAS PUNCHING THE ALTIMETER BUTTON WHEN HE WANTED THE ARPT BUTTON TO DISPLAY ARPTS ON NAV DISPLAY. SO; AS ALWAYS; I NEED TO ALWAYS WATCH THE ACFT LEVEL OFF COMPLETELY TO THE END; ALL THE WAY TO 10000 FT. I MAY HAVE DONE THAT THIS TIME AND THE ACFT MAY HAVE CLBED BACK TO 10400 FT WHEN HE HIT THE ALT BUTTON BACK TO STD; NOT SURE. DID NOT HEAR ANY WARNING HORNS THOUGH IN THAT CASE. AGAIN; COMPLETE THE LEVEL OFF BEFORE ALLOWING MY ATTN TO BE DIVERTED. I USUALLY DO; NOT SURE WHY THIS TIME.