Chapter 18 - "Birthday Surprises" - In which Harry gives Slughorn a bezoar instead of an antidote in Potions class, then asks him unsuccessfully about Horcruxes. The sixth-years then have their first apparition lesson, where Harry overhears Malfoy saying Crabbe has been acting as his lookout, and Harry starts watching the Marauder's Map. Then, on Ron's birthday, Ron accidentally consumes Romilda Vane's love potion and Harry takes him to Slughorn to get an antidote; there, Slughorn gives Ron some poisoned mead and Harry saves his life with a bezoar.
The Bezoar Solution
Without the help of the Half-Blood Prince, Harry is unsuccessful in brewing an antidote to his poison. However, at the end of the lesson, he presents Slughorn with a bezoar instead - delighting Slughorn and infuriating both Ron and Hermione.
The Apparition Lesson
The sixth years have their first [Apparition] lesson with [Wilkie Twycross] in the Great Hall. Harry overhears Draco arguing with Crabbe, but other than that and Susan Bones's [splinching], nothing interesting happens.