Back when I first started I had a guy order two lexan faces for his backlit sign. I drove by, got the measurements of 3'x8', made the signs and he picked them up.
He called back bitching that they didn't fit and he needed them cut down. I told him to measure the old signs and I'd cut them to exactly that size, assuming there was going to be a 1/4" or so difference because the cabinet was warped.
He came back with measurements that I was off 4" in both directions. Being new and not knowing why "I was that far off" I cut the faces down. He called back that afternoon bitching that I'd cut them down too far.
Yep, he'd measured the opening in the cabinet, not taking into account that 2" all around was covered by the retainers. He ended up screwing his new faces into the old faces and just left the lights off.