What happens if you skip this lesson

You'll figure out you needed the exclusion list around week five or six, after you've built two features that should have been excluded and then tried to rip them out.

Ripping out is more painful than writing down. The code is already entangled. The customers who used the feature are already mad you removed it. The lesson costs you a weekend.

The kids who do Lesson 3 properly almost never come back to it. The kids who skip it almost always show up in Module 5 staring at a half-built product they're now untangling. The untangling is the lesson Lesson 3 was trying to teach you cheaply.