Reminds me of over a decade ago I was in a high school AP course. At the start of the course, the teacher recommended we get a AP prep book to study throughout the year, and recommended 2 brands. She specifically said not to get one brand, noting that it was of inferior quality. I had already got the prep book she didn't recommend as I had found it on sale before that remark was made. I later learned that she used that book for all of her test questions, sometimes literally copying the test from the book with minimal editing. I got a 100% on a test out of the blue after averaging in the 80s and then had to make it less obvious.