Mistakes do happen to all of us. An important think I learned is to not jump to far ahead of myself until I have checked out the square before moving forward. It takes a little extra time in the beginning but I don't end up with a large number of 'problem' pieces. As others have suggested set the mishap parts aside and make new ones with substitute fabrics. It might be tedious to re cut the ones that are correct to match those that are incorrect. This would give a smaller end product but maybe you could add more squares or add in some plain ones. I am sure it is usable-just different. Just yesterday someone posted about a project with a major re-do and and how pleased she was for having taken the time to do it.