One of the most infamous such counterattacks was when Something Awful released a "Redditbomb". Said bomb exposed the mods and site management being too cowardly to shut down some subreddits that were distributing child porn. Something similar later happened with r/creepshots on Tumblr, which exposed the personal information of various creepshot posters.
Reddit was also a catalyst for the SOPA/PIPA protests.
Old Shame: Reddit's management have tried to distance themselves from violentacrez as much as possible since the r/creepshots debacle, despite giving him a "pimp hat" award in the past.
Word of God: IamAs featuring media creators, which usually require the celebrity confirming the IamA with a photograph greeting Reddit or some other method.