Since we're on the topic, perhaps some of you can illuminate this scrap from my memory. Back when I worked in a plastics shop, we used to do parts dimension checks with digital calipers. They used to tell us: "If you open 'em up past an inch, close it all the way and zero it before you take a smaller measurement again." The QC lady also hated when people opened 'em up wide, because she said they'd need to be re-calibrated.
Does this make sense to anyone?