Have you ever either run out of bobin while top-stitching a knit hem or been dissatisfied with some top-stitching while stitching around a neck facing? This is an easy way of repairing your stitching without having to remove all of your work. Clip your two top threads with your seam ripper, and pick several out to the right and to the left of the mistake. Do not shorten the threads. On the back side, gently tug on the bobin thread until the top threads begin to show through to the back and pull them through with the seam ripper. Do this on both ends of the unstitched area. You should have at least 1 1/2" of thread tails at each end. Then line up your double needles with the end of your stitching on the top side and fill in the couple of inches of stitching you removed, being careful to end also precicely at the previous stitching. Then tug on your new bobin thread and pull your new threads to the back. Tie the group of threads together at each end. You will not be able to see your repaired stitches from the right side.
This also works very well for hemming jeans or where ever you don't want to bring attention to an interupted line of stitching.