"Myrtle, this is the boys' bathroom!"
-- Harry Potter
There is a boys’ bathroom located on the sixth floor of Hogwarts. One of the mirrors is cracked (HBP21, HBP24). There is a passageway between this bathroom and the marble staircase leading up to the seventh floor (HBP24).
Moaning Myrtle occasionally visits here. This is where Harry duelled Draco Malfoy and Draco was seriously injured when he was hit with the Sectumsempra curse.