Started capturing our family Easter with the 20mm on Aperture priority. Everything fine. ISO stays at 200 as set. Stop checking. (BIG mistake). Switch to 14-140 and aperture jumps to 1250, then 500 and finally gets 'stuck' on 6400 for the next 100 or so shots. I did not go into the menu except to switch off IBIS when I swapped the lens. Somehow by a weird combination of fumbles and random button pushes ISO setting was bumped to 6400. When I got home and downloaded the shots I couldn't believe it but sure enough the menu showed ISO on 6400 despite me having set maximum ISO to 1600 and default to 200 in another screen. Drag out Noise Ninja but the IQ has taken a hammering. Any hypotheses welcome.