I learned about field changes the same way I learned just about everything else about cabinetmaking: the hard way.
The job was a kitchen done is the New Southwestern style with ash, bleached to a sand color and textured by sand blasting. Lots of work went into these cabinets and I wanted to make sure everything fit correctly. The job was complicated by the fact that the main part of the kitchen was an L shape and both legs of the L were bracketed by walls. So no room for error outside of the usual scribing allowance. I measured this job three times just to be sure that there would be no errors.
Considering all of that, you can well imagine my shock when we delivered the cabinets, proudly set them in place, only to discover that both legs were an inch too long. It must have taken me an hour to get my heart slowed down to anywhere near normal. Once it did, I got out the drawings, measurement sheets and a tape, and discovered that both openings were indeed an inch less that my measurements showed.
I could not imagine having incorrectly measured the same walls three times. So I started asking questions and sure enough, the contractor casually informed me that when they had their pre-drywall inspection, the inspector required them to add a layer of 1/2" plywood shear paneling to the entire kitchen, thereby reducing all of the sizes by an equal amount.
We had a pretty good row about who was responsible for the cabinets not fitting and ended up splitting the cost of the remake with the owners. But it left a bitter taste all around and I decided to add yet another clause to my burgeoning contract stating that in such situations, it was the responsibility of the owners or the contractors or the designers to inform us of such changes after measurements were taken.