Ugh I have had the same problem happen more than once (and in fact just yesterday when I washed my just finished quilt....) I have done what Tartan suggested with good results. I don't like to use interfacing - its too stiff - even the light fusible stuff. I just take very small ladder stitiches and as long as I am careful, I can pull the top piece over the frayed one and still have enough solid underneath to ctach without puckering.