Originally Posted by maviskw

I think I know what happened. It has happened to me, but only once. After the binding is put on, I then trim the batt and back. I do this because I want to measure the width of the cut from the sewing line that put the binding on. If I leave 3/8 of an inch from the sewing line, the binding is full of batt and back. But because that fold of the binding is right on the edge, it is very easy to just cut right through it. I think that's what you did.

Others have suggested ways to fix it, but next time fold those corners down and pin them down before you trim.
Good luck.