Fred and George finishing each other’s sentences: this happens less than a handful of times in canon yet in fanfiction they seem to constantly do it. Also in a lot of fanfics they will call themselves Gred and Forge which happens like once in the entire series.

Ron calling Hermione “Mione” he says this once in the fourth book when his mouth is full yet a lot of fanfics have this be some sort of common nickname.

I think a lot of fanfiction exaggerates Lupins comment about Hermione being the brightest witch of her age. He says “You’re the cleverest witch of your age I’ve ever met, Hermione.” he is basically just telling her she is very smart for a 14 year old. Not that she is the next dumbledore.

Dumbledore loving Sherbert lemon/Lemon drops is also something that gets highly exaggerated. Dumbledore simply says he is fond of the sweet. A lot of fanfics have it as his faveourite sweet and constantly offering it to students (bonus points if it is laced with truth serum)