Thankfully has only happened with a wall hanging and a runner. Had enough fabric for the runner back, but somehow got it askewed when quilting. Still sitting in my to do over drawer.
As far as piecing backs, I don't like the look. If you have three equal pieces of fabric for the back, or one large piece that is ok, but more than one fabric to make up for loss to me looks cheap and ill planned. I like the look of the one piece in the middle and one piece on either side of that to equal the full back. Of course, they are all the same fabric so there is consistancy.