"Is this all real? Or has this been happening inside my head?"
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This song's appearance in the film adaptation of Harry Potter and the Order of the Phoenix represents an anachronism, as The Ordinary Boys album on which it is featured, Brassbound, was released in 2005, while the scene in which it plays during the film is set a decade earlier in 1995. In addition, The Ordinary Boys did not get together until 2002, but it is possible that they formed earlier in the Harry Potter universe (possibly coinciding with Britpop phenomenon of the mid-1990s).
Rupert Grint explained that director David Yates chose to have this song play during the scene in the Gryffindor common room in order to create a "more casual feel.[1]