have you ever done this?
 

Well, I got in a hurry again and forgot to change the bobbin thread before machine quilting a small wall hanging. The backing is dark blue and the bobbin thread is white. What did you do? Will the quilt police arrest me if I go over the quilting with a dark blue permanent ink pen?

I don't I have done that before! Your solution sounds good to me! :)

how pretty is the quilting on the backside? - if it is, I'd leave it as white ... especially since it's a wall hanging and no one will ever see the back anyway!

i would leave it also.....on a wall quilt...the backing is always the least of my problems

My machine shows a 'dot' of the bobbin color on top and a 'dot' of the top thread on the back - between the 'longer' stitches - maybe this is what is being referred to?
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I would leave it. You could color it with a permanent marker if you like. I have to tell you (and anybody else that reads this post) The back of my wall hangings and a lot of my table runners are not something that I get too involved with. I just refuse to use a high priced premium fabric for something that is only going to be seen by the wall. I usually choose a solid color and I'm not too concerned if the quilting shows on the backing. My friend and I went to the LQS on Friday and he found a pattern for a large wall hanging he liked. He purchased all the fabrics using the measurements from the pattern (just gave the pattern to the gal that was cutting his fabric) and once we got back to my house, he realized that the pattern called for 3 1/2" yards of backing fabric ( probably 3/4 yd will be used on the front) !! At $11/yard he got a pretty expensive backing fabric for a wallhanging!

I'd leave it.

The 'quilt police' will only know if you tell them. My lips are sealed!

I agree... no one will look, and if they do, too bad!

if its a wall hanging- no one is going to know unless you point it out- so many of our (boo-boo's) are simply never noticed by anyone else unless we make a point in showing/telling about them- if you don't mention it-anyone looking at your quilt will believe it is exactly how you planned it to be- no one knows different unless you make a big deal of it. and since there are no 'quilt police' i don't know what you are worried about-
as far as the marker goes- if your white stitching bugs you & you want to cover it- go ahead- it's yours to do with as you please.