Stupidity: When Ruth catches Chuck fishing for turds in their toilet, he yells at her and asks for privacy and then slams the bathroom door shut. He should have closed the bathroom door in the first place if he didn't want anybody to see what he was doing. It's not like anybody would think he was weird for going into the bathroom and immediately closing the door.
Stupidity: Marta is shown to barf whenever she lies, and Blanc knows this. He should've asked her who did it right away. The entire plot of this movie is rendered unnecessary.
Suggested correction: That would only work if Blanc knew that Marta had knowledge of who killed Harland and if he came to the conclusion that Harland actually killed himself. Blanc doesn't know that Harland killed himself to protect Marta until Marta confesses. Also, this wouldn't have solved the "who done it" anyway, since Marta had no idea that Ransom switched the vials.
Stupidity: When Roy finally gets to the Lima project in the shuttle but can't dock, he just lets it drift away - he could have at least tethered it to the station to use it for his return trip.
Suggested correction: There is no way to know the reason why he did this. It's plausible, or even likely, that the pod was out of fuel. Not only that, but it could not dock with Lima because it was damaged, so it's likely that the damage played into the decision as well.
Stupidity: The marble somehow got under Don's skin at the bottom of his leg and was moving up the length of his body. Instead of heading off the marble in order to intercept and stop it from continuing up his leg, Don got a knife/box cutter and jabbed it into his skin near the bottom of his leg and trailed the marble, causing a long gash that served no purpose (other than creating a lot of blood and making a gory scene). The marble eventually exited his body via his eye. (01:04:40)
Stupidity: When Amy heard Luce coming toward the kitchen, she "hid" his school paper and bag of fireworks in a kitchen cupboard, apparently while knowing Luce would be doing the dishes (and putting them away in the cupboards). Amy and Peter were on the patio (with the door open) just outside the kitchen when Luce was doing the dishes yet Amy made no attempt to prevent Luce from finding his paper and bag of fireworks (which he did). (00:16:20 - 00:21:00)