Stupidity: Ultimately, it did wind up working. However when they shoved the antidote vial into Claire's purse and got George's attention so he would eat her, this was an insanely stupid move. 1. They had no way of knowing if he would actually choose to eat her if he even grabbed her in the first place. Before then he'd been crushing people and even threw the helicopter pilot miles off the building while they watched that happen. 2. The antidote was in a plastic polymer like tube designed to hold the substance inside it while submerged in liquid nitrogen containment. Even if the gorilla's stomach acid could eat through the vial to get to the antidote, it would take hours for it to get through a material like that. 3. Gorillas are herbivores, so it wouldn't have tried to eat a person in the first place. (01:17:55)
Stupidity: After the team break Solomon Lane out of custody, they drive around Paris with him in the front seat of their car. Lane is still wearing a straightjacket, bound in chains and has a black bag over his head. The IMF team drive him through crowded streets, past multiple police cars in full view of everyone just minutes after an armoured convoy transporting a dangerous prisoner (who everyone is now looking for) has been attacked. None of the Parisian police seem to notice this and none of the IMF team questions the tactical soundness of this approach.
Suggested correction: What's stupid about this approach? The IMF team is trusting that with their skills and experience they can easily evade any police that spot them. Which of course they do. Placing Lane in the trunk would have kept him out of sight but that doesn't really matter because they weren't planning on sneaking him out, they were planning on a quick escape.
Stupidity: Why does the team choose to enter the shimmer from Area X and then trek for miles to the lighthouse? They could travel by sea and stage an amphibious landing on the shore right at the foot of the lighthouse. Obviously this would make for a very short movie, but this choice still needs to be explained in the plot.
Suggested correction: I thought it was an obvious tactical choice to go by land. If the mysterious shimmer radiates in all directions for miles, as it seems to in the movie, then it would also radiate for miles out to sea. A recon team is going to have a lot more options to deal with any "weirdness" on the ground rather than on a boat or in the water. (Also, the book "Annihilation" on which the movie is based makes it clear that there are some really big, nasty things swimming around in the water!).
Stupidity: When Han tells Drydon Vos that he and Tobias will steal unrefined coaxium from the mines on Kessel, Vos initially says no, as Crimson Dawn's relationship with the Empire would be at risk. Han then explains that the Empire wouldn't know they were working for Crimson Dawn. Vos then agrees but insists his top lieutenant Qi'ra accompany them, even though she is a known associate of Crimson Dawn and literally has their symbol branded on her wrist.
Stupidity: During the auction, we see the dinosaurs are brought into the room and placed in the middle. Thus blocking half of the bidders from the auctioneer's view. (01:16:40)
Stupidity: The coast cities should have an alarm that detects the Kaijuns. If they do, they should work when the beasts are few miles away from the shore in order to give the people time to get to the shelters. Instead of that there are still people on the streets in Japan and on the docks in Los Angeles (in the flashback) while the kaijuns arrive from the sea.