Stupidity: After stealing the taxi, Cassandra removes the license plate, and yet the taxi's identification number is located in bold plain sight on the doors and roof light.
Stupidity: After so many failed attempts to open the large sea doors, why didn't the apes first breach the facility by climbing to the top of the cliff and simply enter through the existing rusted out exhaust vents where they escaped the rushing water?
Stupidity: During the battle on the harvester, Chani is firing an anti-aircraft projectile at a Sardaukar approaching her. Although the projectile is strong enough to destroy an ornithopter, instead of blasting the Sardaukar away, it pushes it into the air until it hits the harvester track. (00:36:39)
Stupidity: Since we get frequent POV shots from their eyes, we see exactly what the creatures hunting Venom see. Even when the Codex disappears from their view, they can see distinctly Venom's shape, and they have no problem using their appendages and brute force against any and every creature. There are moments in the movie (such as the scene in Vegas) when the Xenophage just stops for no reason; even if the big blinking thing is gone, you can still see the shape it was attached to.