Stupidity: When Drake cuts off power to Hayes' apartment, the cops can't physically make a landlord abide by the laws, and seeing a judge would take months. In the meantime, by keeping the power off in the apartment, Hayes would have had no choice but to abandon his scheme and move on to another victim. With no power in the apartment, it's unusable and unlivable.
Stupidity: After the cloth was lit, the "Santa" could have easily thrown the Molotov Cocktail far enough to ignite the Christmas tree but, instead, casually walked to get closer (within several feet) before tossing it. This may have been for dramatic purposes, but carrying a lit rag (part of a Molotov Cocktail) unnecessarily increases the risk of getting a burn injury (which can be quite serious/painful). (00:02:35)
Stupidity: Joe's second homicide aims to be ruled as accidental death, but Joe's plan involves a furious struggle with the guy underwater, who claws and bites at his arm, as he applies pressure to his shoulder and neck. Moreover, their struggle happens close to the water surface but the bodyguard who is there does not hear or see anything. (00:35:00)
Stupidity: Do the military routinely show up at a civilian's house and then pump thousands of rounds into the house without even verifying if anyone is inside? I've heard of suspension of disbelief, but this scene requires driving it to a vacant lot and abandoning it. (01:11:30)
Stupidity: Near the end, Sydney Wells and Paul Faulkner try to rescue a little 10-year-old girl who is trapped in the recreational vehicle. Sydney is banging on the door from the outside while the little girl is banging on the door from the inside. Paul then smashes out the door window, reaches inside and easily releases the door latch in one move. So, a 10-year-old girl was "trapped" because she couldn't simply open a door latch? Stupid.
Stupidity: Despite the fact that the villains know exactly where Brogan is for the better part of the movie thanks to a tracer, and that they display constantly that they are able and willing to hit with bold military operations even to the point of hitting an urban area overruling law enforcement (apparently Clay has authority to do that too), Brogan is able to get undisturbed back in the US through his friend's plane. Considering he was officially marked as a rogue agent and killed 8 DIA operatives, it's hard to see how Baron's plane could ever land on American soil after a long intercontinental flight.
Stupidity: After Corinne Dufour realises that Drax intends to have his dogs kill her, she inexplicably attempts to flee on foot, running right past the golf cart she arrived in. The golf cart would have given her more speed, range and protection, and would have been a much better choice for fleeing.
Suggested correction: Golf carts are very slow, and are open on all sides: the dobermans would've easily dragged her off it. The biggest stupidity here is that she should've fled to a crowded area with other people instead of the woods. Then again, she was panicking, and not thinking straight.
Stupidity: They say the airline industry was the perfect way to transmit the virus but how would the plane even fly if there was an infected person. In one scene the plane crashed within a minute of an infected person being on it.
Stupidity: David and his partner Cal drive into a weeded area near the garage/warehouse the drug bust is going to be made. David says, "This might go south. We need backup." David and Cal sit in the car talking about not letting the product get onto the street where someone might die, etc. then exit the vehicle without calling for backup, which could have been done in a few seconds (and should have been done as soon as David said they needed backup). (00:02:10 - 00:02:37)
Stupidity: When the police turn up the first time the mother lies to them to get them to go away but the burglars can't hear the conversation. She could have told them exactly what was happening and that she had to make it look like she'd got rid of them. She could have then smashed all the cameras and just let them in.
Suggested correction: This was a character decision, and a completely understandable one. Her daughter was being held hostage in the panic room. She decided to cooperate with their demands for the safety of her daughter. Your idea is a solid one, but she simply did not think of it in her state of mind. She opted to get rid of the police, and her decision is not a movie mistake.
Stupidity: Spoiler alert. After the son makes it out, he could get a rescue team to get his dad by going in where he came out.
Stupidity: When the Woolworth Building collapses and the gang runs into the convenience store to take cover from the dust, Marlena was last seen behind Hud and with Lily, Rob, and Jason. Switch to everyone running from the dust cloud, Marlena didn't make it into the store. Upon exiting the store, the gang finds Marlena stumbling from nearer to the Woolworth Building meaning she would have had to run toward the dust cloud, through the screaming crowd which was running away from the building.
Stupidity: Sam slid under the truck to narrowly escape the lioness. Even though there was more than enough ground clearance, the lioness did not go under the truck to attack Sam, as though the truck's frame were a barrier. The lioness could have easily crawled under the truck (unless a CGI lioness is not as spry or nimble as a "real" lioness). Sam being under the truck would not constitute adequate protection from an attacking lioness. (01:10:30 - 01:35:20)
Stupidity: During the final shootout in Saint's club, Lincoln (the long-haired hitman) sneaks up on the Punisher and fires at his chest with a shotgun at point-blank range, which does not harm the Punisher because of his body armor. Even though Lincoln could see that the Punisher was wearing body armor, he takes another point-blank shot at his chest to no effect when he had a clear head shot.
Stupidity: Given Louise's character, it makes no sense at all for her to leave Thelma in charge of the money. She's known Thelma for years, and knows fully well how flaky Thelma is. And Louise is shown as a person who lives by "If you want something right, you've gotta do it yourself". She's responsible, perhaps even uptight, at that point in the movie. So it defies everything about her character that she would let Thelma guard the money - it smells like a device set up just to allow the money to be stolen.
Stupidity: Peter and Amanda were wealthy enough to buy the 11th floor penthouse for $579,000 but had no security cameras. Also, Det. Martinez never thought to check surveillance cameras outside the Wright's penthouse; surely cameras were around and/or the Detective could have questioned neighbors if anyone saw Charles near the penthouses or with the Wright's dog Humphrey. Peter or the detective could have gone near Charles' boat without a warrant and called Humphrey to see if the dog barked in response.