I used to play the old MasterBook system and will never forget the time when a player used the Betrayal card to screw over another player, while the house was burning down thanks to the Disaster card.
So I recently bought the Plot Twist cards and like the fact that some of them have negative consequences. However, I can't really see the players using them against themselves if they're in their hands. The risk of these cards is that the plot twists will always be twisting in the players' favor if they're only used by players.
What do you think of a rule that if you play a detrimental card during a fight or other pivotal moment, the Challenge Rating is increased? That gives the players a measure of control over the difficulty of some encounters, and can correspondingly receive a reward (XP). Another possible reward would be an extra draw (or two) from the deck.
I'm also wondering about using them as a random encounter mechanic, or to add an element of randomness to basic encounters. Has anyone out there used them as such?