For people who haven't heard about the Holocaust denial thing, this was the interaction that kicked it off
https://gamefaqs.gamespot.com/a/forum/3/31a9d17d.jpg
JKR telling someone "why didn't you check a source for this" about the Nazis destroying the Institute for Sexual Research in Berlin, the world's first sexology institute which documented transgender people and similar matters. Something really easy to Google about.
https://imgur.com/gallery/dXDmJHw
https://www.youtube.com/watch?v=75GL-BYZFfY