
Harry broke his mirror once he realized that Sirius might still be alive if he had used it to check on him before going to the Department of Mysteries at the Ministry of Magic instead of relying on Floo Powder.

Harry only remembered his mirror at the end of Order of the Phoenix, after the events of the Department of Mysteries and Sirius’s death. Harry had decided not to use Sirius’s gift, thinking that he might only get his reckless godfather in more trouble. However, the fact that the mirrors were not used turned out to be a plot device in and of itself. The two-way mirror set that Sirius Black gave Harry was not used in the novel, so the scene was cut out of the film version. In the novel version of The Order of the Phoenix, the pair of two-way mirrors were more important plot devices, and their introduction to the series early on better prepared their later roles in The Deathly Hallows. The movie's omission of a seemingly insignificant detail from the novel where Sirius gave Harry a two-way mirror that would allow them to converse while Harry was at Hogwarts left a significant hole in the usually meticulous storytelling of the Harry Potter universe. This magical item was only vaguely explained in The Deathly Hallows movies because it wasn’t introduced in Harry Potter and the Order of the Phoenix. However, the presence of the mirror, as well as why Harry had a shard of it, how Harry (Daniel Radcliffe) knew that it used to belong to Sirius Black, and how Aberforth knew that he could use it to monitor Harry’s safety were all left unexplained by the films.


Related: Why Harry Potter Didn't See The Thestrals Earlier (It's Not A Plot Hole) Aberforth admitted that he got it from Sirius Black’s house via Mundungus Fletcher and had been keeping an eye on Harry on his journey to find Voldemort’s Horcruxes. In The Deathly Hallows Part 2, they discovered that Dumbledore’s brother, Aberforth, had the rest of the mirror. When Harry and his friends were trapped in Malfoy Manor, Harry used the shard to call for help, and Dobby appeared.

In Harry Potter and the Deathly Hallows, Harry had a small shard of mirror in which he began to see flashes of who he thought was Albus Dumbledore.
