It was an accident, I swear. I honestly don't know how I managed to do this, but I did.
Thank heavens it was at the edge and not somewhere in the middle of the quilt. If it had been I probably would have broken down and cried.
I find trying to cover a larger area usually works better than trying to somehow stitch in a tiny patch that will inevitably fall off. Of course, if I'd had more of the backing fabric I would have tried to match the pattern a bit better, but it just wasn't going to happen.
In my angst and fear I cut out the offending piece of fabric altogether, but thinking back, I probably should have left it there for more stability once it was all fixed.
I did at least have the presence of mind to top stitch it all down once it was in place. In all honesty, barring disaster, it should last as long as the quilt does.
I have, however, learned my lesson and will be much, much more careful when trimming things when the backing is on! How embarrassing.