Parsha Trivia and Riddles

On Tisha B’Av, Chaim learned the laws of Tisha B’Av in the Shulchan Aruch. After the fast, Chaim decided to continue learning the Shulchan Aruch, but now, learning in the simanim that follow the simanim of the laws of Tisha B’Av. After learning for 15 minutes, he lifted his head from the sefer, and exclaimed, “Wow, I had no idea that sometimes there is a mitzva to do a bad job!”

What halacha did Chaim just learn?