B athrooms never change
It sounds stupid, and every person she had ever told had laughed at her for it, but Myrtle had been terrified of using the toilet for the first time in the magical world. Everything was so new and strange, and little things from the clothes to entering the platform to the appearance of food on the House tables was like some big secret that purebloods and halfbloods took for granted. Who knew what kind of odd things the toilet would do in a world where you walked through walls, food materialized from thin air, and a hat could solve your identity crisis?
Even other Muggleborns laughed, because what could be magical about a toilet stall?
To Myrtle, the magic was that there wasn't any. Loos were always the same. At Hogwarts or at home or in the shops in Diagon Alley, it was always the same kind of room. It had stalls you entered by opening the door, toilets that flushed when you pulled a rope or pushed a lever, sinks with regular taps to summon water, and no surprises. Everyone thought it was so funny – that she would think about bathroom similarities and that she found it so comforting.
But when the attacks started, the bathrooms were a place where she felt safe. Other Muggleborns would have envied such a spot. It was where she ran to when she was being teased, and it was the place where she didn't check over her shoulder constantly as if about to see a monster round the corner.
That was why she didn't check when she was coming out of the stall, despite the suspicious presence of a boy and the whisper of a strange password. Anywhere else, she would have suspected the Heir of Slytherin at once.
It's odd that she didn't feel betrayed by the bathroom that held such danger after so many years of lulling her into complacency. The truth is that she was too busy being relieved that, even on the other side of death, the bathroom was the same and offered her the same comfort. Even death could not render a bathroom something different and strange.
There wasn't much else that death and magic left untouched.