"This is a two-way mirror, I've got the other one of the pair. If you need to speak to me, just say my name into it; you'll appear in my mirror and I'll be able to talk in yours."
-- Sirius Black (OP38)
The two-way mirrors were a pair of small, square, old-looking mirrors which allowed secret communication between the two owners. They belonged to Sirius Black in the 1970s. He and James used to use them when they were in separate detentions (OP38). Sirius gives one of them to Harry at the end of Christmas break in 1995 with the words “Use it if you need me.” He tells Harry it’s a “way of letting me know if Snape’s giving you a hard time” (OP24). Harry never thinks of using the mirrors until after Sirius’ death. He takes the package out of his trunk and reads the message on the back:
This is a two-way mirror, I’ve got the other one of the pair. If you need to speak to me, just say my name into it; you’ll appear in my mirror and I’ll be able to talk in yours. James and I used to use them when we were in separate detentions (OP38).
He remembers seeing his parents in the Mirror of Erised and tries desperately to contact his dead godfather using this mirror. It doesn’t work, and he throws the mirror into his trunk and breaks it (OP38).
About a year before the Battle of Hogwarts, the mirror which Sirius had owned was stolen from Number Twelve Grimmauld Place by Mundungus Fletcher. It finds its way into the hands of Aberforth Dumbledore when the Hog’s Head barkeep buys it from the thief. He had been told about the mirror by his brother before his death (DH28). Aberforth uses the mirror to keep an eye out for Harry Potter and he hears Harry ask for help when trapped at Malfoy Manor. Aberforth sends Dobby to the rescue (DH23).
Sadly, if Harry hadn't ignored this gift from Sirius and had remembered to use it to contact his godfather before charging off to London to the Department of Mysteries, the entire battle would have been avoided. He would not have been caught in Umbridge's office, the core group of Dumbledore's Army wouldn't have been lured into the clutches of the Death Eaters, and the Prophecy would not have been recovered.
And Sirius wouldn't have died at the hands of Bellatrix.
Harry's "saving-people thing" flaw is compounded by his "don't trust adults" attitude. Both flaws lead to dire consequences in the saga.