Dumbledore was really regretting eating 5 burritos earlier. It really wasn't sitting well with his bowels.
Of course, it didn't help that he couldn't find any of the bloody bathrooms.
After walking past that one tapestry of the idiot who thought teaching trolls to dance ballet was a brilliant idea, Dumbledore spotted a door he hadn't seen the first couple of times. Shrugging, he pulled it open to find… a room full of chamber pots. All with unique patterns painted on them, some even with a bit of gold plating on the sides. Dumbledore didn't care, he just needed to shit.
So, he sat on the nearest chamber pot, and let out an audible groan as his bowels emptied themselves into the pot. Then he realised something.
There was no toilet paper.
Fortunately, just as he thought that, he spotted a roll a few feet away. He pulled out his wand and summoned it to him. Unfortunately, it was only a one-ply. Damn, budget cuts, he thought, tearing off some to wipe his butt.
Later, he could never find the room again.
I offer no explanation