Every theologian, wanna-be theologian, a-theologian, and otherwise thinking person has one.
Discuss a point of theology long enough, and you'll inevitably see it played. Call it Anderson's Law: as a theological conversation grows longer, the probability of seeing the mystery card approaches one.
You'll learn to see it coming. The shoulders shrug just a little, a sympathetic smile starts slowly forms, slow-motion starts as the words hit you: "Well, some things are a mystery..."
This is a dangerous card for the theologian to play, as it functions as a bit of a trump card. Play it too early, and you short-circuit the difficult process of coming to a more robust understanding of the subject of inquiry. Don't ever play it, and end up like Chesterton's lunatic who tries to get the heavens into his head, only to have his head split.
With that said, here are a few of theological and a-theological frameworks and the distinct places where the mystery card gets played:
Yes, the list is a bit of a joke. But it's a joke to tease out the difficulty of knowing where to place our mysteries, and how many we should admit.
But seriousness aside, this is a game we can all play. Add mystery cards in the comments and I'll update the post accordingly. Bonus points for picking on your own tradition(s).