You can apparently also flag a comment by mistake, harder to do it with a post, but not impossible. Happened to one of my comments earlier today and I was really wondering why did it get a downvote, so I've asked and got a reply that it was a mistake. Good thing that you can retract your vote and fix mistakes like that and that goes not only for downvotes, but for upvotes as well.

I have also received what should be considered a revenge downvote from people that I have accused with a comment for trying to post fake information and pretend they are somebody else here on steemit. This is enough of a confession for me to return the "favor" and flag them back. So no censorship here, just getting rid of the posts and users that have no place on steemit for an adequate reason.

Now, it is also possible that somebody can purchase a lot of Steem, turn it into Steem Power and start abusing his enormous voting power as a tool for revenge against somebody else or against steemit for some reason. That can actually be a real problem...

I guess that some people are concerned, because steemit is being promoted as no censorship social media among other features. That however does not mean that anything should be allowed, SPAM for example has no place or fake people trying to cheat users and run away with some money if they manage to.