Not aligned Dado shelves - does it need correction? (With pictures)

This is your project, so in the end, it is your call. You have a few options:

1. Do nothing! Try a test fit and see it if looks ok. If so, move on to the next step! Generally speaking, a 2 mm difference over half a meter/a few feet will not jump out at the viewer (unless there are other errors). Nor will it compromise the structure of a bookcase (a forgiving design).

2. Do almost nothing. You can insert thin wedges under the shelves (on the side with the larger dado). Again try a test fit - it will work but you may not like the look.

3. Recut the smaller dado - make sure you pick the correct side of the dado to cut! You want dados at the same height and size to avoid shelves with a slight diagonal across the case.

4. Use a hand tool to trim the small dado - depending on your hand skills and hand tools this can be quickly cleaned up with chisels or special hand planes. Again, you want to cut the correct side of the dado - the shelf should install with right angles to both sides of the case.

I would advise against trimming the shelf to match the two different dados but you can make that work as well.

Ensure that the shelves are secure when assembled - that is important. :)