Missing Data – Textbook Chapter

I had a class with an instructor who had a modified textbook for our course that covered only 3 chapters of the original book, but on a test, they had accidentally included a 4th chapter in the questions because they were using the full textbook. It made me feel like I was missing something vital, but since I read my whole textbook, I couldn’t figure out why I didn’t know it! (The instructor took those questions out afterward and apologized)