I've heard people say you can mix sawdust from your workpiece with wood glue to make an invisible patch, but I've tried it twice (once in oak, once as pictured on ash) and both times it came out significantly darker and the surrounding wood. The glue is Tite Bond 3.
Does a different glue work better? How wet should the sawdust paste be? This is on the back so it doesn't matter a lot, but I'd like to have a good process for filling little mistakes.
Also I just went back and read your original question. The trick to this saw dustpatch method is to use very little glue. Start with just a dot of glue right on top of the boo-boo spot and then lightly dab it into the spot with your finger. Then take a piece of sandpaper that is on the more used side (simply because the glue will be picked up by the sand paper and won't be good for much else after this operation) and lightly sand with the grain. The spot that is partially filled with glue should accumulate the freshly sanded (color matched! Booyah!) dust and form a near invisible patch. Then allow to dry for a bit before applying a finish over top. I'll see if I can find a video of the technique.