For me it's usually when I am cornered into making a quilt for some one who chose colors or different types of fabric that just don't play nice together. And I always try to be extra careful with my cuts when it's their fabric, because I have no idea where they purchased it. One lady brought me a pastel pink satiny and a bright safety orange printed duck cloth to make a baby quilt. When I tried to explain why these were not compatible she said just do the best you can with it. Well I put it back in the bag and handed it back to her and said this is the best thing I can do with these. Reasons 2&3 were I could not inflict a baby with those colors or the rough feeling of the duck.