Stupidity: Long after the Marine Life Institute has closed for the night, the recording of Sigourney Weaver welcoming visitors is still playing.
Stupidity: At the end of "You're Welcome", Maui puts Moana in a cave and placed a large rock blocking the only known exit. This means, as far as Maui knows, he's trapped a human in a cave with no food, water or exit as he didn't know Moana could make a very lucky escape. Basically Maui was going to leave Moana to starve to death so he could take her boat.
Suggested correction: This isn't a mistake. Maui doesn't care about anything other than escaping at this point. He even says, Without his hook he is nothing. He's not interested in returning the heart at first so even though he is a demigod, he has lost interest in being good. Another example is when he leaves Moana to face TeKa on her own.
Stupidity: After Noah shoots the Humanity Bureau vehicle following them, they continue down the road and stop at an old abandoned building that still has some sort of running water in it. Lucas has to go to the bathroom so his mother just lets him wander off by himself to find a restroom. She doesn't go with him or make sure he's fine. And this is in an area where they know that most of the residents are hostile, and they are being chased by the Humanity Bureau. Noah even calls her out on the stupidity of her letting Lucas go by himself and she claims it's because he needed to do number 2 and that Lucas insisted and he's not a little kid any more... Which doesn't excuse the stupidity of the action of letting an 11 year old go by himself to find a bathroom in such a situation. And ultimately it does lead to him getting found by Agent Westinghouse. (01:08:30)
Stupidity: Project Pegasus is a billion dollar structure with no security guards besides the couple dudes Fury shows the badge to at the entrance, no video surveillance, and once SHIELD arrives nobody has to even open a locked door anymore.
Suggested correction: First of all, Fury was allowed in as it's a government facility and he works for the government and as a SHIELD agent is allowed access. It's inside a mountain and they passed multiple security guards as they drove in, armed guards. Everything is thumb prints and cameras which was quite elaborate for that time. To say they lack in security is quite an understatement. SHIELD has quite some authority and can easily take control in the Pegasus project facility.
You are right about the main entrance being truly secure both for guards and strategic position. The problem is that they are free to just roam the facility for an hour, blast through doors, not a soul in sight, no evidence of camera monitoring the inside of the structure, and once the gag of the pad is finished, no door requires it. Actually, funnily enough you can see a guard of the place opening the elevator for Fury and the supervisor, as if the thumprint scan was needed to even get into the elevator itself, but Keller then just walks into the archive just fine (from a different door than the one Vers blasted). There are keypads to exit places (for instance the hangar, when the agents in pursuit break through the door you can see a keypad on the wall) but only when it's convenient (Vers and Fury walked through that same door with no problem, not to mention the fact that the whole stairs seem to have none, which is funny for a place that has keypads both sides of doors).
Stupidity: The X-Men depart for space on a vehicle not designed for space, for an incredibly dangerous mission they know nothing about (and none of them can survive in space at that point), but don't suit up for it in the slightest - and with Nightcrawler and Quicksilver's powers as they are shown later, it'd be easy to do it without wasting time. Not only that, but Xavier communicates with NASA to get the briefing only after the X-Men have already departed.
Stupidity: The Good Boys learned about consent in assembly and knew about making a citizen's arrest, using the internet to find where the drone is sold, how to treat a dislocated shoulder, etc. but did not know that they shouldn't give out their address on the internet and invite a stranger into the house?
Stupidity: After making love, Selina notices Batman's scars. She surely was caught in the moment, but, it's something she really was bound to notice sooner. (00:53:10)
Suggested correction: She notices the scars on his back first, which are the worst of those on his body, this may well be the first time she sees his back.
Stupidity: The marines all wear helmets as part of their protective gear when they go into the Phobos compound, but Bennett and Akua do not wear any helmets at all. They wear bullet proof vests and arm themselves with weapons, but unlike the other marines, they go in without any way of protecting their heads.
Stupidity: So if Harley Quinn drove the truck to blow up the plant, wouldn't she have had to drive through all the cop cars at the bar murder scene? We know it was the same street because the cop gives the necklace to the detective. (00:10:58)
Stupidity: Right before the movie's final battle, Sub-Zero taunts Cole with his wife's necklace and makes him chase him into a portal. Sonya was right next to Cole, and she did not come along with him. They had previously planned to take on Sub-Zero together, but apparently she just left him to chase Sub-Zero alone to an unknown location.
Stupidity: Royce, blinded in Afghanistan, managed to avoid serious injury when the boat finally hit land. Having no real idea where he was, he foolishly decided to get off the boat and try venturing through unknown woods with the dog Chinook as a guide (!) This was simply absurd! Even though rescue missions were delayed that day by the storm, he should have had enough sense to know his chances of being found/rescued (and being safe in the meantime) would be the greatest by staying in the boat.
Stupidity: Luca's parents try to find him by splashing him with water. If they succeeded, Luca's cover would have been blown.
Stupidity: Despite being wanted for murder, escaping jail, and having his face plastered on the front page of the newspaper, Morbius meets Martine in a public diner and doesn't even make an attempt to conceal his identity while they are seated. Even shaving off his beard and cutting his hair short would have gone a long way in making him more unrecognizable.