Stupidity: Arishem's plan with the Deviants hardly makes any sense; he sent out biological beasts that he can't control, to kill off the natural predators of every planet. Forgetting the fact that obviously it didn't work (but the movie does not say that) because Earth has always had predators, that's a terrible plan to begin with, since any ecosystem needs predators or the other animals will grow uncontrollably.
Stupidity: In the scene around Tenochtitlan, the Eternals have a device that allow them to locate the Deviants - they know for sure that they have exterminated every breathing one. In the rest of the movie this technology is never brought up again. It also makes it really odd that it took them 6,500 years to kill the Deviants, apparently never actively hunting them.
Stupidity: When Thena suffers from her first spell of Mahd WY'ry, Ajak knows what it is and flat out tells her that they have the technology on the ship to wipe her memory off to start over. Somehow none of them asks themselves any question about any of this. They all think that this mission was their first mission, but for the plot to work everyone in those boring 6500 years together must have never talked to each other about their past - Sersi accepts instantly the fact that her home planet doesn't exist, so she doesn't seem to have any strong recollection about it. And nobody ever inquires Thena about those memories of other planets ending up in disasters.
Stupidity: Phastus' partner learns with astonishment about the giant-shaped rock in the Indian Ocean from the TV and asks "You guys did that?" He must have lived under a rock to be asking that in a scene that takes place two weeks after the events and without a doubt has been all over the news ever since. (02:16:45)
Stupidity: The Eternals are in a rush to figure out what is happening to the world and reunite the group. So naturally they go to their spaceship, which also happens to be where the speedster of their group is, as last part of their trip. Ikaris (which admittedly has an agenda of his own, but the others don't know it and know his abilities) is with them from the beginning, can easily fly across continents and even makes it to the Sun from Earth in a comically short time at the end of the movie, but instead uses their comparatively slow spaceship to spread the word.