It is season 4 episode 18. Here is a summary of the episode
http://www.mtv.com/news/2244348/catfish-fake-victim/
I can't remember the exact reactions but Machine Gun Kelly was a guest host and was not happy about it. I imagine the producers where pretty happy that they got something exciting happening.
That's brilliant. I couldn't find anything about this episode on google, but I'm interested to know how the hosts reacted to that.