I've used a black sharpie to colour grey thread between black fabric pieces. I'vee used wonderunder on a little piece of fabric cut to cover the wrong colour in a paper pieced pattern. It was a small center piece that threw off the picture pattern.
I've threaded a hand needle with the same colour of thread and fixed a long machine quilting stitch boo boo. I used the needle to make pretend connecting stitches so it looked like the threads connected.
I've used pliers to disconnect the head of a pin from the shaft to remove a pin found in a finished quilt. The head stays in but the shaft comes right out.