I do as others have mentioned - go over the last inch or so of the previous line, perhaps with a couple of backstitches, too. The backstitches sometimes show, though. I figure better that than that the quilting comes undone in the wash.
Also, for purely cosmetic boo-boos, if you are using cotton batting for that wrinkled look, when you wash it, the shrinking and wrinkling will cover a multitude of sins.