For well-intentioned posts that are factually incorrect, it seems that there are usually replies that explain the reason for the downvote. When a comment is wrongfully banished to the grey nothingness, oftentimes others step up and defend it. The community immune system functions well as is.
Also, I adore the fade-to-grey side effect of the downvotes. It’s like spritzing a cat with water for scrounging on the counters—an ultimately harmless but super effective reprimand.
Such a moderation comment might be anything, but the more egregious examples:
Illegal content (of some sort); A threat upon persons or property (E.G.); Hate Speech; other things that a reasonable person would agree do not belong with a discussion; or just being Wildly off-topic.