Out for a walk today I shot a series of phtographic masterpieces on my Pentax K-S2 fitted with a Tamron 18mm-250mm zoom. When I got back I found some where very under exposed, generally/always the first in a bunch of shots. The Exif info shows that the under exposed and correctly exposed shots have the same/very close shutter speed & aperture (I was shooting mostly on Aperture Priority.)
My hypothesis is the aperture mechanism was a bit sluggish, possibly due to the low temperature (around 3C), so took some time open up to the required stop on pressing the shutter.
Does that sound reasonable? What can I do to stop it happening again whwn out and about in the cold?