Errors in measurements are not always the fault of the quilter doing the blocks or project. Once I needed to make two blocks as part of a group scrappy project for an ill quilt guild member. I found this really neat block pattern to be made from paper templates. The instructions clearly stated to use the templates as is...no requirements to reduce or enlarge on the copier. Well, I made the first block only to have it come out like a bowl instead of laying flat.
I double checked and it turned out the templates were not correctly sized and I had to redraft the templates. Moral...it is not a bad idea to test the template pattern with either paper or scrappy pieces before starting the actual project.