i recently did the same thing. i finished a top and laid it on the floor. it was perfectly flat and square. taped it to the wall. same thing. when i quilted it i did heavy quilting in the center and less on the border. SHOOT ME PLEASE. there is nothing i haven't tried to get the border to lay flat again. the binding is on and i have it pinned to a wall, tying to stretch it out. the next time i do heavy quilting in the center, i will remember to bring it all the way out to the edge, or heavy-stitch the border in a different pattern. live and learn.