What's something from Canon that fanfiction exaggerates to comical degrees?
Harry being some kind of mini Gordon Ramsay.
A lot of fanfics portray Harry as some kind of amazing cook under the assumption that the Dursleys forced Harry to cook every meal. In the books the only times we see Harry doing any cooking for the Dursleys are when he fries some eggs and when Vernon tells him to watch the bacon. I think the opposite is probably true, Vernon and Petunia would probably not trust Harry to cook anything complex. Harry probably does know the basics of cooking but the idea that he is some expert chef is a bit comical.
Fanon nicknames.
Gred and Forge pop up a lot in fics as if it's their common nicknames, they refer to themselves as that once in the book, and the entire context of that is that they are making a joke about the initials on their sweaters. Similarly things like Ron calling Hermione "Mione" happens once and it's in the context of him having his mouth full. I also don't think anyone ever refers to Harry, Ron and Hermione as the "Golden Trio" the closest parallel I can think of is Snape very sarcastically calling them the "dream team."
Dumbledore calls everyone by their first name.
In the books the only people Dumbledore ever regularly calls by their first names are Harry and Voldemort. Harry is the student he interacts with the most and he almost certainly calls Voldemort "Tom" to rile him up, besides I doubt Voldemort would be any happier if Dumbledore ran around calling him "Mr Riddle."
Hogwarts letters always come on your birthday
Hagrid showing up during Harry's birthday was pure coincidence, and it's also specifically mentioned that Harry had been receiving letters in the weeks running up to his birthday.