Battlestar Galactica is a board game in which three to six people work together to negotiate crises, manage political and military responsibilities, and fight off enemies in space combat. There are two sides: the humans, and their rebelling robotic creations, the Cylons. Each player is dealt a secret loyalty card. Roughly one third of the players are told they are on the Cylon team, either at the beginning of the game or part way through the game. The Cylon players must find ways to sabotage the human players' effort, yet keep from being discovered, lest the human players put them in the Brig and limit the amount of influence they have in the game. Ultimately, a Cylon player can reveal himself as such and gain access to a different set of abilities, but it's beneficial (and much more fun) for the Cylon player to remain hidden, working their evil plans while denying their true loyalty.
It's quite normal for the accusations to start flying early in the game, as the players find bits of evidence of sabotage during the course of crisis management, in which players contribute help or sabotage anonymously (though careful deduction can point to the saboteurs). Near the end of the game, if no Cylon players have revealed themselves, it's usually because they have a particularly devious plan up their sleeve, and it becomes even more important for the human players to identify and oust the Cylons. The paranoia rises as the humans near their destination, and victory for their side.
So the mechanic for identifying the traitor players is the loyalty deck, which is composed of cards that say whether or not you're a Cylon. Each player gets one card at the beginning of the game, and another one halfway through the game. If a player got a You Are Not a Cylon card at the beginning of the game, but received a You Are a Cylon halfway through, they have to switch sides immediately. Even the players have a certain trepidation about this event. The deck is constructed with enough cards so that about a third of the players will get a You Are a Cylon card.
What I really love about the game is the emergent trash talk, accusations, and analysis of player behavior. You draw Politics cards but you say you can't help us negotiate this crisis that requires Politics? You must be a Cylon. You're hoarding your Quorum cards instead of using them to improve Morale? Sounds like a Cylon to me. You just repaired the FTL Control room instead of our damaged Vipers? Toaster loving fracker, to the Airlock with you.
One time I screwed up and didn't add the You Are a Cylon cards to the loyalty deck, and everybody got You Are Not a Cylon cards. As the game progressed and we were all honestly playing for the human side, we became frantic as we realized that nobody appeared to do anything bad. The game was going quite well for us, but we became driven to find the traitors, and death threats were made over the smallest suspicious infractions; actions that were heroically helpful became reasons for indictment because of their superlativeness. I gave a few open suggestions of how a Cylon player might try to influence the game for their side, just in case one of the players was confused about what they should do, and for my efforts, the Admiral stripped me of my Presidency in a military coup. Of course, this made me think the Admiral was a Cylon. My girlfriend, who can usually guess which side I'm playing, didn't know what to think.
When we won the game and all revealed our You Are Not A Cylon loyalty cards, our fervor turned to sheepishness and shame.
It was truly the most interesting game of BSG I've played, even considering that the actual goal of winning was easily obtained (and was pretty boring without any opposition). The inter-player conflict that the game so carefully crafts was pitched to a level that the rules alone could never create (helped along by gin, tonic, and resentment - always a good time). The only problem is that now everybody suspects me of spiking the deck, and I could never get away with it again.
But it's the thought that counts.