Similar to a troll, a forum rat is someone who has spent so much time on an internet forum site, such as Gametrailers or Gamespot, that they start to use the usual forum-like mannerisms in real life. Whether intentional or not, this is not a good thing. It could have originated from term mallrat.
Person A: "Dude, I totally caught this 20 pound marlin out near Cabo San Lucas last weekend"
If someone on a forum website starts a thread or makes any kind of questionable claim, someone will probably reply to the post saying "Pics or Fake". As in, "post pics to prove that you are telling the truth, otherwise you are lying". %90 of the time this does not provoke the intended outcome, and goes relatively unnoticed by the OP.
Original post: "Seriously, one time I ran into Tina Fey at a bar in New York."