I learned that there is no escaping destiny. Measure twice cut once? Pah.
I was making a door for the pallet shed under construction (I'll dig out a photo in a bit). I spent forever calculating the position and size of the diagonal brace, and its exact angle. Pythagoras came out of his box, Corel draw scale drawing in wire frame mode to validate the calculations. Cut the pieces with m millimetric precision, having taken account of the saw blade width.
Everything fitted together perfectly, and square ( possibly an issue with a pallet shed). Only, the top rail was 150 mm lower than it should have been.
Just can't win.