Currently, there are some advertising ethics mistakes such as one-sided pursuit of influence, deliberated distortedness and rights infraction of thinking of children and image of women.
He wrote in a letter, and I quote, 'It is not difficult by concealment of some facts, and exaggeration of others to bias a well-meaning mind, at least for a time'.
This might be poetic licence, but I think it more likely that Beedle had only heard about Animagi, and never met one, for this is the only liberty that he takes with magical laws in the story.