Mass isn't conserved. The sun operates by destroying matter, as do atom bombs. Energy, which matter is a form of, is conserved. The violation isn't the turning into a cat, it's the lack of a subsequent mushroom cloud.

First all, I like this story. I think it's rather well written. But that statement in the story almost made me close my browser. Harry gets rationality in the modern-muggle world, but is very resistant, and expresses complete unwillingness to except something in the magical world until he can prove it. I feel like if I were in the story and I saw this version of Harry Potter, I'd be calling him an idiot every 4 sentences he says. Just because you don't understand something, doesn't make it impossible.

To quote Men In Black:

"Fifteen hundred years ago everybody knew the Earth was the center of the universe. Five hundred years ago, everybody knew the Earth was flat, and fifteen minutes ago, you knew that humans were alone on this planet. Imagine what you'll know tomorrow. " ~ K

It was a very, very dense cat.

E=mc^2