I had a number of shelf brackets mounted on a wall, wanting to add three pieces of shelf on top. Unfortunately the brackets were a little bit off, so the shelves looked awful where they met. 3D-print to the rescue! These little shims, printed in 1 and 2 mm heights, were enough to align the shelves nicely.

The default values fit the smallest and now out-of-production brackets of a common German type, but can relatively easily be adjusted to fit the larger ones.