The card you were already holding

Terror is three descents, and finishing one well lets you carry a card into the next. It is a reward for a clean finish, and it is meant to be the same physical card travelling with you.

Being the same card is the whole rule. Two of one card is not something these rules can express, so a carried card has to leave the deck it would otherwise have been dealt from.

It left one of them.

What was actually happening

The next descent is built from two piles: the Gathering you draw from, and the Pantheon the entities come out of. The carry looked in the Gathering, found the card, removed it, and stopped.

Number cards live in the Gathering, so for most carries this was correct and invisible. An entity’s card does not. It lives in the Pantheon, so carrying one put it in your hand and left it standing in the Pantheon as well.

The result is a descent containing two of a card that exists once. You would reach an entity whose card was in your hand. And because a card can be spent to settle a Manifestation, it was possible to pay with the copy you were holding and leave the game in a state it has no rule for.

Why nothing caught it

Every test of the carry used a number card, because that is what a carry usually is, and against a number card the code was right.

The tests are better than that now. There is one that carries an entity’s card specifically and asserts it appears exactly once in everything the next descent is built from, which is the property that actually matters rather than the one that happened to be exercised.

What changed

The carry now removes the card from wherever it lives. If it is an entity it comes out of the Pantheon, and that descent is one entity shorter, which is correct: you are holding it.

I am sorry to anyone whose Terror run ended somewhere that made no sense. It would not have looked like a bug. It would have looked like the game had lost track of itself, which is exactly what it had done.