Stupidity: Beck wants to kill Peter's friends because they know his secret. Instead of using Edith to attack them directly with a drone strike, or using his illusion technology to lead them into the path of a train like he did with Spider-Man, he instead has a henchman drive them onto a bridge and leave them in the path of his next Elemental attack. Because absolutely nothing is forcing them to stay on the bridge, they all casually walk off the bus and out of immediate danger. It is unfathomable that a man as intelligent and resourceful as Beck would take such an idiotic approach, especially considering all he had at his disposal and how desperate he was.
Suggested correction: He wanted it to seem like they were killed in the Elemental attack because it was cleaner. If they were killed by a drone it would be much more suspicious than being killed in the disaster. Once the plan goes wrong, he does simply send an Edith drone after them. If it wasn't for Spider-Man's timing, he would have been successful as well.
But that is the major problem, and why I think it was an egregious mistake in the movie. The plan "goes wrong" because it was idiotic. So idiotic that it is unrealistic that Beck, a highly intelligent person, would have made such a glaring oversight. Leaving the kids on the bridge but not trapping them at all allowed them to make an easy escape.
I think the point is that Beck thinks he is the smartest person in the room and that this plan is going to work. Should he take into account MJ and co's free will, yes, but he is so maniacal (and not thinking rationally) that it does not cross his mind. This is proven by the fact that as his plan is failing around him that he still wants his suit pressed and ready to meet the Queen because it will work out in the end in his mind. Also, to your point, having them walk in front of a train or walk off the bridge, would not make him a hero. He needed real casualties and Peter's friends were the place to start. Finally, in the sequence showing Beck and his team preparing for the attack, he was focused on the theatrics of the attack and, again, thought the size of it alone would work (he wanted it bigger, scarier, more forceful).
Suggested correction: Fury is well aware of the drone system (he berates Peter for misusing it earlier). If Beck simply utilised EDITH to kill the students, it would give away that Beck was using the drones for his own gain. Once Fury was dead, he could have used EDITH had the original plan failed, but he certainly couldn't do it until after Fury (and potentially other SHIELD agents) had been taken out. He was going to attack London no matter what, so he took the opportunity to take out Ned, MJ and Betty at the same time.
This doesn't stop him from using a targeted drone strike to kill the kids, he was planning on using it to kill Fury anyway. The fact that he fails in his strike against Fury is irrelevant to the fact that he needed those kids dead and decided to take a round-about way of accomplishing this goal. Again, he doesn't have to use a drone strike, he is perfectly capable of using the illusion technology to force the bus off a cliff or into some other immediate danger. Having a henchman drive the bus to a bridge and hope the kids are dumb enough not to escape danger when literally nothing is forcing them to just stand there and be killed is ridiculously idiotic.
Suggested correction: Characters, even intelligent ones, are allowed to make bad tactical decisions. Real-life history is replete with examples. Just because it seems unlikely doesn't make it a plot hole.
True - this was originally submitted as "stupidity", which is slightly different, but this seemed like such a massive oversight that it qualified as a plot hole.
Suggested correction: Beck's intentions were to make it look like the kids were killed in the attack by the monster. Had he just killed them with a drone out right, it would have obviously looked like murder and foul play bringing in more investigations and potential problems for him.
But again, he doesn't need to use a drone strike he can use the illusion technology to trick them into an accident. Even what he chooses to do (just leaving them on the bridge) would have also been fine had he trapped them there at all. Just leaving them there without trapping them is so stupid it is unbelievable. It's like leaving someone on train tracks but not tying them up.
Suggested correction: He was an insane person and wasn't thinking fully rationally.
Stupidity: They make a big deal about how they've got the bare minimum of Pym particles left as Hank has been snapped away, but it never occurs to them to use their supply to jump back to a very safe time when Hank was around with a vast supply of them. They could bring them to their time and then have no end of attempts to get the stones without being on such a knife-edge.
Suggested correction: The problem is that they need to steal particles, and if they fail doing that, everything fails. They never thought of doing it that way because of that. Only when they failed in the past did they have to risk that, with again the possibility of failing. They couldn't take the risk to have their only chance of reversing what Thanos did fail because they want to be leisurely about it, ironically. They could do it in one go, that was the best bet and lowest in risk.
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).