My old boss in Milwaukee said skip the cardboard and just build it, and boy was he wrong
When I mocked up my new dog wash station, my old boss Dale said cardboard was a waste of a weekend and to just cut the real plywood. I ignored him and spent 3 hours with a $2 box and some hot glue, and good thing too, because my first layout put the tub 8 inches too close to the wall and I would have wasted a $60 sheet. The cardboard let me move the whole thing twice before I ever touched a saw. So what's the worst layout mistake you caught in a mockup that would have cost you real money?