I let people know when they call to bring a quilt in that the backing has to be larger and I explain why. If someone brings one over and I find it is too small I call them right away and find out what they want to do. Sometimes I can baste muslin on to have enough to load, sometimes they want the top trimmed, sometimes they replace the backing or bring over fabric for me to add. Every situation is different but the key is Communicating.