Note the electric guitar
An anachronism is person, object or idea that, in fiction, occurs in the wrong time.[1]
There are many in My Immortal, particularly in the chapters where Ebony visits the 1980s.
In My Immortal[]
Right from the beginning, it's clear that the story does not take place in the 1990s, when Harry Potter canonically takes place, but rather the mid 2000s, with references to My Chemical Romance, Evanescence, Good Charlotte, etc.
Tara has an iPod, despite not being invented until the early 2000s (it wouldn't even function within Hogwarts due to the magic interfering with most electronics). Characters also have mobile phones, which while those did exist during the '90s were not commonly used by teenagers, and once again would not function properly within Hogwarts.
There are many references to movies and TV shows that also were released long after Harry Potter canonically ended (1998), such as Corpse Bride, Underworld, Resident Evil (assuming she is referring to the Milla Jovovich movies instead of the original video games), and more.
In I'm Not Okay[]
Similarly, Raven's story also has many anachronisms, such as iPods, music from the 2000s, Emily the Strange, etc. It would appear that the reviews she got complained about this, as in Chapter 3, her author's note began with:
"look, I don't care when the f Harry Potter is in my version it takes place now."
In Ghost of You[]
In Raven and Tara's collaborative fic, the author's note includes a very similar sentiment:
"I don't give a f**k when HP takes place. This takes place NOW, and it's MY story, so they can have iPods and NORMAL clothes and sh*t if I want them 2."
Naturally, they do have such items.