Been there!!
It's happened a few times even.
Especially if I'm trying to make a smallish piece of fabric "work".
Usually, I end up being 1/2 or 1" short along the final bit of one edge.
So, I have become the self appointed "Queen" of fudging it. I just add a strip onto the backing, trim and press the seam well, and quilt over it. Most of my backings are patterned fabric and unless you look for it, you don't see it.