Stupidity: When Wolverine is trying to prevent Deadpool from sacrificing himself by destroying the time ripper device, he repeatedly pounds his fists against the glass panel on the door to break it, to no avail, which leads him to then go to another heavy door, which he is only able to break down in the nick of time. Wolverine's adamantium claws would have easily destroyed the glass on the other door, without him having to repeatedly and futilely punch it.
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: When an explosion hits the courtroom, Arthur is the closest person to it, with nothing to protect him, but he is the first and only one to recover, and every bailiff (not to mention the police outside the courthouse) outside of explosion range is mysteriously knocked out and/or absent.
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: 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.