Anti-Seki
The official commentary gives this position as example where both black and white stones could be considered dead:
If both players pass, while leaving the above position on the board, then all of the black stones and the single white stone are considered dead, however, they are NOT removed from the board for scoring.
The white stone is considered dead, since it can be captured by a hypothetical move at A. Black is dead, since white can play at B and then A to capture, since black cannot use any ko threats win the ko fight (see later post about the special ko rules that apply only during the confirmation phase).
Despite being considered dead, they do not count as prisoners and white does not get any corner territory, since they are not removed. The commentary makes this clear by saying:
If the game ends like this, Black and White are both dead but none of the stones can be removed. According to Article 8 there is no territory. Compared with playing A, Black loses 3 points.
I find this to be confusing and unclear how it follows from the seemingly relevant articles in the rules:
Article 8. Territory
Empty points surrounded by the live stones of just one player are called “eye points.” Other empty points are called “dame.” Stones which are alive but possess dame are said to be in “seki.” Eye points surrounded by stones that are alive but not in seki are called “territory,” each eye point counting as one point of territory.
…
Article 10. Determining the result
1. After agreement that the game has ended, each player removes any opposing dead stones from his territory as is, and adds them to his prisoners.
So, it seems that the explanation might be that despite the stones being dead, they are not removed since the corner is not considered territory, which is the case since the points are either: 1) not considered surrounded by the live stones of white, or 2) the points are considered dame? I’m not sure.
Note: the phrase “anti-seki” is not coined in the official commentary, but is used on Sensei’s Library. This article also explains the double-ko seki position as an example of anti-seki.