Ae you sure you're casting off correctly? There's a good description of casting off, with pics, at http://www.knittinghelp.com. THe only other thing I can think is once you work the cast off, you'll have that last stitch that your last cast off stitch goes over. That stitch is part of the flap (either back or front, depending on which side you're workign on) You have to actually work that stitch with the flap, it's part of the stitch count that I've got in teh pattern. If you just leave it hanging and don't work it, then essentially it will unravel the whole cast off edge.