Alright Harry Potter fans, time to put your detective hats on and determine if these fan theories should be canon or not. We've all reread the series so many times that we basically know more about it than J.K. Rowling, right? Prepare to have your minds blown...
1. Hagrid is a Death Eater:
"Early on in the books, I noticed the strangest behavior coming from Hagrid. I started paying more attention to the actions, statements, and inconsistencies in Hagrid and realized almost every move taken somehow aided Voldemort. By far the majority of the evidence points toward Hagrid as a servant of the Dark Lord. Hagrid has been in the service of Lord Voldemort at least since the First Wizarding War, potentially since his time at Hogwarts. Hagrid has been performing deep cover tradecraft, espionage, source validation, sabotage, and spotting for Voldemort. Hagrid is secretly a much more talented wizard than he lets on. Hagrid is secretly much more talented of a liar than he lets on. Hagrid is a high ranking servant of Lord Voldemort.
In the Sorcerer’s Stone, we see how Hagrid passes private security information directly to Voldemort. He also subtly manipulates Harry to put him in direct contact with Voldemort. He is pretty casual about offenses that would get him placed in Azkaban. The magical abilities Hagrid uses are otherwise only known by high-level Death Eaters. In the Chamber of Secrets, Hagrid spends time in areas known fro dark wizards and witches without explanation. Hagrid also is strangely familiar with Voldemort and has known him for many years. Int he Prisoner of Azkaban, Hagrid purposefully spreads misinformation and rumors, which only aid Voldemort. He also has access to all of Voldemort's enemies and targets.
In the Goblet of Fire, Hagrid saved Voldemort's wand, illegally cheated in the Triwizard Tournament, and conspired with Barty Crouch Jr., disguised as Mad-Eye Moody. In the Order of the Phoenix, Hagrid demonstrates how he is a covert actor, rather than the bumbling oaf he normally acts like. In the Half-Blood Prince, Hagrid seems to systematically sabotage Harry's education. Finally, in the Deathly Hallows, Hagrid only seems to fight Voldemort and the Death Eaters when it is obvious they will lose. Hagrid also collects valuable intelligence information for the Death Eaters in preparation of the Battle of Hogwarts."
Read the full reddit thread for even more details, it is truly wild.
2. Nagini is Voldemort's mother:
"In Half-Blood Prince, we find out from Mrs. Cole that Merope arrived at Wool's orphanage, gave birth to Tom Riddle, and died soon after, though I believe Merope faked her own death. Perhaps Merope was looking ugly due to her upcoming permanent transformation into a snake. Perhaps Merope gave her child to an orphanage before faking her own death and running away so she would not harm her child after her snake transformation. The Gaunts after all are descended from Salazar Slytherin - what better family to carry a snake curse?
If this were true, and Voldemort was the child of the maledictus Nagini, it would explain Nagini's maternal relationship with Voldemort in the books. If the two were able to communicate via parseltongue, this theory would explain why Voldemort is so fond of Nagini, entrusting her to become a Horcrux. Voldemort drinks Nagini's venom to regain strength in his baby-form in Goblet of Fire, an obvious parallel to mother's milk. It may even explain why Voldemort develops snake-like characteristics. But craziest of all is how this might explain how Voldemort was able to recover to a baby-form at all. Maybe the reason Voldemort was able to regain a baby-form was that his own mother, Nagini, gave birth to him again. As Nagini contained a piece of Voldemort's soul, maybe Voldemort's soul joined with Nagini's piece of soul, which would later be given a body through childbirth."
h/t to reddit user u/soupycutie
3. Professor McGonagall is living in a time loop:
"Dr. Minerva lives her life in a loop, exactly ninety times. Her inner animal is a cat, and she sought the form to be able to de-age unnoticed. What confuses most people is that she is supposedly born in 1935, but she's in Hogwarts as of 1927 and 1910 (when Leta and Newt from Fantastic Beasts are 13). Minerva was born in 1789. After marrying, she became a mother and grew old. Around the 1870s, she had the happiness to see her granddaughter be born: Isobel Ross. But shortly after, she became aware of her de-aging condition. She distanced herself from her family and went to Hogwarts. She taught a few classes while de-aging, and she also sought information about what was happening. You see in the Fantastic Beasts sequel removing points from Leta Lestrange in 1910 as a Professor. It's not a constant de-aging, and it depends of factors such as stress or her physical condition. In the remaining 17 years, she gets younger by roughly 30 years. Of course, the only person who knows exactly what is happening is Albus Dumbledore. He therefore offers her a position as assistant to not raise suspicion.
By 1933... When she's soon becoming a baby... She performs an incredible piece of magic to be reborn through her Grand-daughter Isobel Ross. At age 11, She starts at Hogwarts in 1947 and finishes her studies 7 years later in 1954. In December of 1956, she starts as a Professor at Hogwarts again and remains there for 39 years. She also survives the second Wizarding war and is currently 83 years old by 2019. In 6 years time, she'll start de-aging again. She sought to become an Animagus so that she could hide while de-aging. She couldn't keep being reborn out of descendants or other wizards."
Seriously, check out the rest of this jaw-dropping fan theory here:
4. Harry's poor eyesight is due to Voldemort's attack on him as a child:
"The question has been brought up many times: 'If wizards can heal all sorts of horrible injuries and regrow bones, why can't Harry fix his eyesight and avoid having to wear glasses?'
Well, we know from George's injury in the Deathly Hallows that some wounds caused by dark magic can't be healed or fixed, why is why they were unable to save George's ear after losing it to Snape's curse. Is it possible that some side-effect of Voldemort's rebounding curse damaged Harry's eyesight beyond magical repair? The Killing Curse normally leaves no physical markings, but Harry was the obvious exception. We know from his and Hermione's visit to the house in Deathly Hallows that the room was partially destroyed as well. Being that Harry was only one at the time, we really have no indication if his eyesight was poor before the incident, but is it plausible that his glasses, almost as much a trademark as his scar, come from the same incident."
Reddit user u/BoredOneNight has more details here: