This unsightly notch was from me trying to make a cut with my miter gauge despite it being far too short to support the piece. Had I been smart and built a properly sized crosscut sled, I would not have made this mistake. (Note to self, make panel sled)

First step is to make it worse, for real. In this case I took a chisel and straightened up the cut so it was flat and even. What I didn't document was that I took a thin offcut from one of the stretchers and cut a corresponding angle to match the grain direction.

Take that offcut and glue in place. Make sure you get it tight against the shelf and some good pressure with the clamps. 

You can see how creative I had to get since I couldn't fit a clamp between the slats.

After the glue was dry I used a flush cut saw to trim the edges the a block plane to bring it flush with the rest of the shelf.

I then took a sharp chisel and did final cleanup on the edges and cleaned up the entire edge with my low angle jack. Make sure you're careful exiting end grain here.

If you don't have one, a Low Angle Jack will amaze you in its ability to deal with end grain and hard wood.

Speaking of endgrain. Always make sure you clamp a backer board to your end grain pieces when planing. This provides the necessary support to keep the grain from blowing out on the exit cut.

This is my waste block, but you can see the blowout that will happen on an unsupported edge. If this happens to you, put some liquid CA glue (Thin CA)) in the fibers and clamp them back in place. Once that is dry you can clean up the area with proper supports in place. (Or just going the other direction if you can)

Remember when I said that the shelf dados went mostly to plan? The one that didn't was the middle: it was a hair too wide. I could have likely glued this with good clamping pressure and left it, but I wasn't satisfied with that. 

I wanted a good tight joint, so I took some of the thin maple offcuts and planed them to a wedge shape. I cut these to length just over my leg thickness and used a mallet to drive them in tight as I was gluing up the shelf. This locked in the shelf and kept the top of  the dado tight.

You can see both wedges here.

Once I had everything wedged into place and the glue was dry, I used a marking knife to score the edge and was able to break the wedges flush.  It hides these gaps nicely, even if they're on the bottom and will never be seen by anyone who isn't an over obsessive woodworker.