Slughorn's Christmas Party

"I don't think you should be an Auror, Harry," said Luna unexpectedly.
Everybody looked at her. "The Aurors are part of the Rotfang Conspiracy, I thought everyone knew that. They're planning to bring down the Ministry of Magic from within using a combination of Dark Magic and gum disease."
Harry inhaled half his mead up his nose as he started to laugh. Really, it had been worth bringing Luna just for this.

-HBP, UK edition, p.299.