I place the quilt top on my design wall then begin to place the backing on top remembering to leavie enough for a longarm quilter. I too have made mistakes and here is my most recent one. I had two extras fabric squares left over so I attached a border around each one (they are NEVER printed square) to sew into the backing. But on the background piece that is below and above the block, I was short over an inch!! So I just insert a long red strip that is going down on the left block and going up on the right block. Thank goodness that problem was resolved. It probably looks a little bit more interesting than what I had originally planned. So don't EVER feel like you are the ONLY one making mistakes !!! Hang in there!!! Hopefully there will always be an easy fix for any error.