That happened to the me on the quilt I am making for my grandson's high school graduation. I had measured perfectly. But, when I was making the sandwich I started one corner off just a little bit and it shifted a little bit. By the time I got to the bottom, the backing was off about an inch and a half. I sewed on another strip-but it off on the other side which was now too wide. I had used Elmer's school glue so there was no undo and redo! I can hardly see it and by the time I finish quilting it it will disappear completely. I am not one to point out my errors to others so I am sure that I will be the only one to know. But, that video is helpful so thank you for posting it.