Andy's correct observation seemed like a good time to explain what some of the issues that can cause such shading/distortions and how they can be corrected.
Auto smoothing works very well and is handy in the early stages of a build, but as Boof has correctly mentioned in his tutorial, there are rules that have to be obeyed for the best results.
There is also a comment on cleaning up the excessive verts that usually result from using the boolean cutting tool. Another handy tool, but remember "clean up" is required.