It's not a mistake- it's a design opportunity! And you are not alone. I bought enough fabric to back a baby quilt. I didn't want to make a creative back with pieces from the front, i just wanted to seam it and be done. I bought 2 lengths. In my haste or absentmindedness, i cut it too short! So I had to make a creative back with pieces from the front. But guess what? I had enough of that backing to make another creative back for another baby quilt. Sometimes it turns out even better. Good luck.