FLC OF AN LTT INADVERTENTLY CLBED ABOVE ASSIGNED ALT.

Narrative

IN CRUISE FLT AT 16000 FT; FO WAS FLYING. ZFW CALLED WITH A PLT'S DISCRETION DSCNT TO 11000 FT AND A NEW ALTIMETER SETTING CONSIDERABLY HIGHER THAN WHAT WAS SET. WHEN I CHANGED EITHER THE SELECTED ALT OR THE ALTIMETER; I DO NOT KNOW WHICH ONE CAUSED THE PROB -- THE ALT HOLD MODE ON THE AUTOPLT DISENGAGED. I WENT BACK TO FILLING OUT THE LOGBOOK AND NEITHER I NOR THE FO; WHO WAS FLYING; NOTICED THE ACFT BEGIN TO CLB. WHEN THE ALT ALERTER SOUNDED; WE BOTH DISENGAGED THE AUTOPLT AND NOSED OVER; BUT THE ACFT HAD ALREADY GONE THROUGH 16600 FT; NO DOUBT ALERTING THE CTR CTLR WHO; ALTHOUGH HE DID NOT REQUEST OUR ALT OR MENTION IT IN ANY WAY; ORDERED AN IMMEDIATE DSCNT TO 16000 FT. THIS IS A CLASSIC CASE OF TRUST IN ACFT SYS TO THE EXTENT OF NOT FOLLOWING THROUGH WITH A CHANGE OF SETTING WITH OBSERVING THAT THE SYS WAS PERFORMING PROPERLY BEFORE LOOKING AWAY FROM IT. THERE WAS NO EXCUSE FOR ME TO NOT ENSURE THAT THE ALTIMETER AND ALT SETTINGS DID NOT AFFECT -- AS THEY SOMETIMES DO -- THE AUTOPLT AND THERE WAS ALSO NO REASON FOR US TO NOT NOTICE THE ACFT BEGINNING TO CLB.