A CLBING MD80 OVERSHOT ITS ASSIGNED ALT OF FL280 BY 1200 FT WHEN THE ALT ALERTER WINDOW WAS SET TO FL290 VERSUS FL280.

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I WAS PNF ON AUG/TUE/98; WHICH WAS IN PROCESS OF LEVELING OFF AT FL290 1000 FT ABOVE FL280 ASSIGNED. CTLING AGENCY ZAU RECLRED FLT TO FL280 WHILE CLBING TO FL230 ON MKE-STL LEG. THE CAUSE OF THE OVERSHOOT; I BELIEVE; WAS A COCKPIT PREOCCUPATION WITH THE CAPT'S PROB TRIMMING THE ACFT PLUS A POSSIBLE MOVEMENT OF THE ALT WINDOW DIGIT FROM 28000 FT TO 29000 FT AFTER THE ALT WAS SET. TO MY KNOWLEDGE I SET AND ARMED 28000 FT AND POINTED TO THE WINDOW TO DOUBLE CHK AND THEN TO THE YELLOW 'ALT' READOUT ON THE MD80 FLT MGMNT ANNUNCIATOR WINDOW. DURING THE CLB THE CAPT ASKED ME SEVERAL QUESTIONS ABOUT THE ACFT TRIM AND IN MY RESPONSE I MENTIONED THAT I USUALLY JUST TRIM WITH AILERON AND WORRY ABOUT RUDDER TRIM ADJUSTMENTS AFTER LEVELOFF. HE CONTINUED TO EXPLORE THE TRIM PROB UNTIL I FELT THE ACFT LURCH FORWARD IN PITCH AND I SAW 29200 FT ON MY ALTIMETER. IF 29000 FT WAS MISSING IN WINDOW IT SHOULD'VE LEVELED OFF AT 29000 FT SO I BELIEVE SOMEHOW ALT SETTING SLIPPED 1 DIGIT AND DISARMED THE ALT WITHOUT PF OR PNF (ME) SEEING IT. RECOMMEND: 1) CHK WITH MANUFACTURER TO SEE IF ALT WINDOW CAN 'AUTONOMOUSLY' SLIP 1 DIGIT AND DISARM AFTER IT HAS BEEN SET AND ARMED? 2) PLTS BE ENCOURAGED NOT TO DISCUSS ANY NON ESSENTIAL SUBJECTS WHILE ALTIMETER IS MOVING; ESPECIALLY DURING THE INITIAL CLBOUT WHEN ACTIVITY SLOWS DOWN.