The Harry Potter Fan Theory That Pinpoints Slughorn's Biggest Mistake
Throughout J.K. Rowling's Harry Potter series, there are numerous parallels between the hero Harry Potter and the villain Lord Voldemort. They're both Parselmouths, able to speak the language of snakes. The core that powers their wands came from the tail of the same phoenix. And Harry has a certain arrogance — a tendency to act like the rules don't apply to him — which Dumbledore observes early on as a shared characteristic with the teenage Tom Riddle. It's this last commonality that perhaps lends credence to an interesting fan theory.
If this fan theory is to be believed, Harry once took advantage of a teacher in the same unethical way that Tom Riddle did during his time at Hogwarts. The theory, posited by Reddit user The-Nightblender on the Harry Potter subreddit, is that Tom Riddle used the potion Felix Felicis, a.k.a. "Liquid Luck," to persuade Professor Horace Slughorn to tell him about Horcruxes in exactly the same way that Harry used Felix to get Slughorn to reveal the truth of his Horcrux discussion with the young Voldemort (it's one of many interesting details from Harry Potter and the Half-Blood Prince, like this Voldemort family connection).
"I just find it odd how willing Slughorn was to tell a student how restricted, dark magic works, and how to do it," The-Nightblender wrote. "And also how he knew crystalized pineapple was Slughorn's favorite. It parallels to how Harry convinced Slughorn to get his memory, and convinced him to accompany him to Hagrid's hut, as well as the fact that he KNEW Hagrid's hut was the place to be."
Liquid luck
In Harry Potter and the Half-Blood Prince, the sixth book in the series, Harry wins a vial of Felix Felicis from Hogwarts' new potions professor, Horace Slughorn, who had previously held the same position for many years, and taught Tom Riddle as well as Harry's mother Lily. Slughorn has a crass habit of cultivating close relationships with promising students, such as the celebrity Harry Potter. As Dumbledore describes it, Slughorn has a need to "collect" witches and wizards who stand a chance of becoming famous in the future.
Riddle had once been a member of this "Slug Club" of promising students, likely another reason Slughorn was so willing to provide the young Voldemort with information about the vile dark magic that would make him nigh invincible. In Harry's day, Slughorn is ashamed that he helped Voldemort, and tries to keep their illicit conversation a secret. He even goes so far as to alter his own memory of the event. But Harry, under instructions from Hogwarts headmaster Albus Dumbledore, uses his clout, the memory of Lily, and a little help from Felix to get Slughorn to tell the truth about what he told Riddle fifty years prior.
In order to get the memory he needs from Slughorn, Harry drinks the Felix Felicis in order to get good luck, a kind of performance-enhancing drug for accomplishing magical tasks. He runs into Slughorn on the way to Hagrid's cabin by "happenstance," setting off a chain of events that ultimately leads Slughorn to reveal the true memory of that fateful day in his office with Riddle.
Riddle me this
The-Nightblender's theory is that Tom Riddle did almost the same thing to get Slughorn to tell him what he wanted to know about Horcruxes. As the theory goes, Riddle took the potion, which furnished him with important information about the professor's favorite candy. Redditor u/HufflinPuff, ran with The-Nightblender's theory, adding that the timing of Harry and Riddle's use of Felix Felicis lines up. "If Slughorn generally gives the Felix Felicis prize to his 6th year NEWT students, Tom would have been in his 6th year or 16 years old and undoubtedly would have won the prize," they wrote.
User calledannie thinks this theory makes sense for Slughorn's character. He feels shame about what he did, he just doesn't handle that shame well. "I've always felt a little sorry for Slughorn, the guilt he must feel after Riddle became Voldemort. This theory offers him a little humanity, but would still make sense why he (Slughorn) would feel guilty and not necessarily understand why he was so open to discussing it with Riddle," calledannie wrote.
Another user, lightningblazes, disputed the theory. "I think Slughorn just really liked Tom Riddle," they wrote. "He was exceptional and in his own house. Tom Riddle wrote the book on charming people to get what he wants. Slughorn also surrounds himself with what he likes so it wouldn't be the most difficult thing to guess that he likes crystalized pineapple. Harry was not at that level. Harry himself acknowledges the skill employed by Riddle to extract the information."
It's a good theory, but there's no way to know if it's true short of shaking down Rowling herself. Whether you believe it or not probably comes down to whether or not you're willing to give Horace Slughorn the benefit of the doubt.