A MEDIUM TRANSPORT TURBOJET FO RPTS UNCOMMANDED SWITCHING OF ALTIMETER SETTINGS FROM CORRECTED TO 29 PT 92.

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FLT ABCD FROM ZZZ TO ZZZ1: THE BEFORE START CHKLIST WAS ACCOMPLISHED; WHICH INCLUDES CHKING ALTIMETER SETTING AGAINST INDICATED ALT BTWN CAPT'S SIDE; FO'S SIDE; AND STANDBY INSTS. BOTH THE CAPT AND FO ARE INVOLVED IN THIS ALTIMETER SETTING PART OF THE BEFORE START CHKLIST. ON THIS PARTICULAR NIGHT; THE ALTIMETER SETTING WAS 30.51. ZZZ DEP CLRED US TO 10000 FT. UPON LEVELING NEAR 10000 FT; ZZZ DEP STATED THAT OUR ALT WAS 10500 FT AND THE ALTIMETER SETTING WAS 30.51. I LOOKED AT THE ALT TAPE AND IT WAS INDICATING 10000 FT; BUT THEN I NOTICED THAT THE ALTIMETER SETTING WAS AT 29.92. (THE CAPT WAS TALKING TO COMPANY WHILE ALL THIS HAPPENED.) I RESET THE ALTIMETER TO 30.51 AND IMMEDIATELY WENT DOWN TO 10000 FT. ZZZ DEP CONFIRMED 10000 FT AND SWITCHED US TO ZZZ1 APCH CTL. SOMEHOW; THE ALTIMETER SETTING HAD BEEN CHANGED TO 29.92 WITHOUT KNOWING IT. THERE IS A BUTTON ON THE KNOB THAT CHANGES THE ALTIMETER; WHICH; WHEN PRESSED; SETS 29.92. I DID NOT; AS FAR AS I CAN RECALL; EVER TOUCH THAT KNOB AFTER THE BEFORE START CHKLIST WAS COMPLETE. SO; IT'S A MYSTERY AS TO HOW THAT ALTIMETER SETTING CHANGED. LATER THAT NIGHT; IN A DIFFERENT ACFT; FLT EFGH ZZZ DIRECT ZZZ2; I SELECTED ZZZ2 ALTIMETER 30.46 FROM 30.47; BUT THE ALTIMETER SETTING SKIPPED 30.46 AND WENT TO 29.92 AND THEN 29.91. ALL THAT OCCURRED WITH JUST ONE CLICK OF ALTIMETER SETTING KNOB.