Arthur Weasley made the Weasley family status clock
The Weasley Family Starus Clock is easily one of the most interesting magical objects shown in the movie. It is featured more in the books, but its origins are unknown. The fans think that Arthur Weasley is the one who made it for his wife, Molly.
This fan theory points out that Arthur is fascinated with artifacts and objects, even from the muggle world. He is a skilled tinkerer and a powerful wizard, which could allow him to create something like this. The books also clearly state that nobody else has this kind of device.
Dumbledore also knows about the clock, referring to it as “wonderful”. We also know that the Weasleys have a lot of family members who also travel a lot. It could be true that Arthur made this or had someone make him a device so they could keep track of the family members in times of crisis.
Voldemort killed Harry’s grandfather
The main argument for this fan theory is that, after his parents get murdered, Harry did not go to Lily’s parents. We know that James’ mother and father died before Harry was born, but we never hear anything about the Evans family.
Those would be Lily and Petunia’s mother and father. The only mention of them was when Petunia says they approved of Lily being a witch. This further strengthens the possibility of them accepting Harry.
So, the fans think that the only reason this did not happen was that the Death Eaters or Voldemort already killed them when they were searching to break the FIdelius Charm. That charm conceals the location of the targeted person/s in the soul of a secret keeper.
James Potter sabotaged Snape and Lily’s relationship because of jealousy
Snape and Lily met before Hogwarts, while they were still kids. They developed a strong bond and presumably love. At Hogwarts, the love continued, but James wanted Lily for himself, as this fan theory goes.
So, he intentionally orchestrated numerous humiliations and provocations for Snape, which made him push Lily away from him. According to this theory, James ruined their relationship as he knew that, if provoked enough, Snape would push Lily right into his arms.
Snape intentionally didn’t make Lupin’s Wolfsbane Potion strong enough
In the third book, Harry Potter and the Prisoner of Azkaban, Lupin teaches at Hogwarts. He transforms into a werewolf and causes some trouble, but Dumbledore knows about this, and he gets Snape to fix Lupin the Wolfsbane potion.
It is supposed to stop the person drinking it regularly from shifting. Some fan theories suggest that Lupin forgot to drink it the night they all went to the Haunted Hause and uncovered Peter Pettigrew’s deception.
But, some fans think that Snape intentionally didn’t make it strong enough as payback for all the bullying the Marauders inflicted on him back in high school. And, Lupin was a part of the Marauders.
The Dursleys were mean because they were near a Horcrux all day every day
By the end of the Harry Potter story arc, we learn that he was the seventh Horcrux, the one he never intended to make. It is doubtful that even Voldemort knew this up until the very end because he himself destroyed it.
However, he made Harry his Horcrux when he killed his parents when Harry was still a baby. We know how Salazar’s locket affected Ron when he wore it, and Horcruxes corrupt everything in their environment. Thus, the Dursleys were so mean to Harry because they were near a Horcrux for over a decade.