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.