The Mishnah teaches that a situation can arise in which the Kohen Gadol himself disqualifies his own grandmother from eating Terumah. How is that possible?
The chain of generations before us:
The grandmother - a daughter of a Kohen who married a Yisrael: Through her marriage to a Yisrael she lost the ability to eat Terumah, and she bore him a child. Even if her husband died, as long as she has a child from the Yisrael she does not return to her father's house to eat Terumah.
The daughter - a daughter of a Yisrael who married a Kohen: Her situation is precisely the reverse of her mother's. Her mother was the daughter of a Kohen who married a Yisrael, while she is the daughter of a Yisrael who marries a Kohen, and she bore him a son.
The son - a Kohen: The son is a Kohen like his father, and he is even fit to be a Kohen Gadol, since every Kohen is fit for the High Priesthood, to stand and serve at the altar.
This son enables his mother to eat Terumah while she is alive, even after the death of her husband, the Kohen. This is the rule: when the husband who was a Kohen died and left a child behind, that child enables his mother to eat Terumah.
However, if the mother dies, that same Kohen Gadol turns out to disqualify his mother's mother from Terumah, since he is a descendant of the grandmother's Yisrael husband. Any descendant she has from the Yisrael prevents her from returning to her father's house to eat Terumah - and so he disqualifies his grandmother, even though he is the Kohen Gadol.
And this grandmother would note the irony and say: "Al yehi adam k'vni zeh" - let no man be like this son of mine - meaning her grandson, the Kohen Gadol - "haposeil osi min haTerumah" - who disqualifies me from Terumah, for it is precisely because he is her descendant through the Yisrael that he disqualifies her.
In summary: We have learned the case in which the Kohen Gadol himself disqualifies his grandmother from Terumah - a daughter of a Kohen who married a Yisrael and bore him a daughter, and that daughter married a Kohen and bore a son. While the mother is alive, the son enables her to eat Terumah, but once she dies, his status as a descendant of the grandmother's Yisrael husband comes to the fore, and through that he disqualifies her from returning to eat Terumah in the house of her father, the Kohen.