Head-canon you’ve accepted that is not supported by, or directly contradicts, canon
I believe the best answer here would be the names of Hermione’s parents, Harry’s grandparents and Gringotts’ director. The reason I’m choosing this over any other head-canon I’ve accepted is because most of my head-canon is fairly fluid according to the type of story I am reading at the time. The names, though, are something that I always, always believe, to the point that I feel wrong-footed when they are different.
In the Harry Potter books, Hermione’s parents are, by and large, kept in the background. As such, aside from their alias’ after Hermione wipes their memories, their first names are never revealed. I don’t know who was first, but a lot of fan-fiction writers, particularly Harmony shippers, use the names Dan and Emma Granger; a fitting tribute to the actors who played Harry and Hermione, Daniel Radcliffe and Emma Watson. These names have become so entrenched in my mind that I feel uncomfortable when I see them called anything else.
Similarly, Harry’s grandparents are never named in the books. Unlike Hermione’s parents, however, Rowling has named his paternal grandparents on the Potter family tree on Pottermore. According to this, Harry’s grandparents go by the names Fleamont and Euphemia Potter. To me, though, they will always be Charlus and Dorea Potter. Again, I don’t know who used these names first, but they have been around long enough (well before Fleamont and Euphemia) and are used often enough that they have become fanon.
Ragnok was named in Harry Potter and the Order of the Phoenix as a goblin that Bill unsuccessfully tried to recruit into the order, refusing due to his belief that the Ministry was covering for Ludo Bagman’s unpaid debts. His role within the bank was never mentioned but many fan-fiction authors place him as the Gringotts Director. I have seen this in quite a few stories but I truly accepted it as head-canon after reading Harry Crow by robst, one of my favourite Harmony stories.
I do have other pieces of head-canon, of course, but, as I stated earlier, they tend to change somewhat depending on the type of story I’m reading; a Weasley bashing time-travel story as opposed to a good Dumbledore Harmony story, for instance.