This is delicate. First, someone who mentioned a result to a famous mathematician, and received some feedback, might decide to add the famous mathematician as a coauthor after their death, without giving that person a chance to protest. It is hard for journals to distinguish such situations from genuine collaborations.

Second, an author might decide to keep a manuscript in their drawer because they are not quite sure it is right. If it is then published after their death, it can lead to further contradictions. I was told that a story like that is behind the paper

Pontrjagin, L. Characteristic cycles. C. R. (Doklady) Acad. Sci. URSS (N. S.) 47, (1945). 242–245.

The reviewer (Hassler Whitney) wrote in https://mathscinet.ams.org/mathscinet/search/publdoc.html?r=1&pg1=MR&s1=13317&loc=fromrevtext that ``Both theorems contradict a statement of the reviewer [Proc. Nat. Acad. Sci. U. S. A. 26, 148–153 (1940), near the end of §4; MR0001338]. It is not easy to find who is wrong where.''

I was told (but have not verified this) that the mistake in Pontryagin's paper was due to reliance on a posthumously published paper by Elie Cartan.

This is delicate. First, someone who mentioned a result to a famous mathematician, and received some feedback, might decide to add the famous mathematician as a coauthor after their death, without giving that person a chance to protest. It is hard for journals to distinguish such situations from genuine collaborations.

Second, an author might decide to keep a manuscript in their drawer because they are not quite sure it is right. If it is then published after their death, it can lead to further contradictions. I was told that a story like that is behind the paper

Pontrjagin, L. Characteristic cycles. C. R. (Doklady) Acad. Sci. URSS (N. S.) 47, (1945). 242–245.

The reviewer (Hassler Whitney) wrote in https://mathscinet.ams.org/mathscinet/search/publdoc.html?r=1&pg1=MR&s1=13317&loc=fromrevtext that ``Both theorems contradict a statement of the reviewer [Proc. Nat. Acad. Sci. U. S. A. 26, 148–153 (1940), near the end of §4; MR0001338]. It is not easy to find who is wrong where.''

I was told (but have not verified this) that the mistake in Pontryagin's paper was due to reliance on a posthumously published paper by another mathematician. (Edit: As noted in the comments, this could not be E. Cartan.)