Personally, I wouldn't care one way or the other. If the quilt was all finished, then the boo-boo probably wasn't that noticeable to begin with, and even if it was a gift, there is no way I would take it apart just to fix one little mistake.
While I don't make it a point to create errors in my quilts, I actually think a little slip-up now and then just adds to the charm of a hand-made quilt. None of us is perfect, and our creations are not perfect either. It's the meaning behind the quilt that matters the most.