Stupidity: Bond has been shot at and hunted down by armed men and dogs. He must conclude his life is in danger, and yet after Bond despatches the armed man in the creek who is carrying a British Sten submachine gun, he fails to pickup the weapon which has far superior firepower to his small calibre hand gun. Had he done, so he may well have saved Quarrel and avoided capture.
Stupidity: The "keep everyone in the stadium" plan is foiled when gold is scattered around the outside and everyone leaves. The bad guys didn't consider locking the doors? Not least because once the terrifying sky tendrils descend and boiling red clouds start filling the arena, surely people would have started bolting for the wide open exits.
Stupidity: Even though Hermione reminds them that they must not be seen after freeing Buckbeak, they fly toward the castle, whooping and cheering because they're flying. Then, they land in the courtyard of the castle, have a conversation, and then Sirius flies off with Buckbeak. Not the behavior of trying to remain unseen and unheard.
Stupidity: Luca's parents try to find him by splashing him with water. If they succeeded, Luca's cover would have been blown.
Stupidity: When Jonathan is chasing the truck to try and save Sarah, he could have removed his heavy backpack and run faster...but instead just leaves it on. No wonder he wasn't fast enough.
Stupidity: When the flying snowmobiles attack, Bond tells Elektra to go into the gully, and he'll lead them into the trees...completely ignoring the fact that these bad guys are here to kill her, so why would they follow him instead? Of course they all do. Either a plot contrivance or a blatant set up, which he doesn't remotely consider.
Stupidity: Very soon after Damon is stranded, one of the many satellites around Mars inevitably picks up the fact that he has moved various objects around his site, indicating that he's still alive. Yet it never occurs to him that this would obviously, eventually happen. In which case, his best option would be to simply spell out a message with debris and stay put instead of planning some 50-day journey.
Stupidity: Darren could have easily grabbed Ben out of the locker he climbed into by just pulling. Instead, he pushes it inward, causing it to close and allowing Ben to either hold onto it on the inside or lock the locker itself.
Stupidity: Two oil rig workers were looking at two young ladies in bikinis who were sunning themselves on a boat/yacht. One oil rig worker said, "That's where we should be right now. With them, enjoying the sun, instead of baking in it." Both men had double layers on - a jacket or sweater over a jersey or T-shirt - and one man was wearing a tossel cap. Even if it gets cold on the rigs at night, they did not need to be dressed in warm attire during the sunny daylight hours. (00:10:35)
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.
Stupidity: Jake uses several Loops and travels all over the world in order to get back on the boat at the end of the film. This was completely unnecessary as only the final Loop in 1942, which was available to him in 2016, was the one he needed to enter despite the fact that its nature should have prevented him from reaching 1943 anyway.
Stupidity: After leaving the church in the painted Studebaker, the duo have Max and Hopper right behind them. Therefore, there's no way the (conveniently) colored car could possibly fool Max and Hopper.
Suggested correction: That was the whole point. It was supposed to be funny.
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).