Stupidity: No U-boat commander is going to "finish off" a ship that is dead in the water and is on fire from stem to stern. He exposes his boat and his men to mortal peril by surfacing, brightly lit by a full moon and the fires burning on the sinking ship, making them a sitting duck for the destroyers that might well be still in the area. He also wastes a precious torpedo for absolutely no net gain.
Stupidity: Major Franklin is struggling up the side of the cliff while the rest of the group are on top of the cliff checking their equipment, then he slips and slides down the cliff side, breaking his leg. After seeing this, the group members reach for a long length of rope that was used to bring up the other members and the supplies. The commandos could have used the rope to help Major Franklin climb up and prevent him from falling and injuring himself.
Stupidity: The lack of security at the château is not plausible. The Germans typically would have layers of protection around headquarters or other sensitive areas. In the movie, there are two middle aged guys guarding the road into the Château. After that there are no checkpoints, no patrols, and no controlled access points into the castle. It makes no sense that dozens of officers would be left vulnerable to an attack by French resistance fighters, let alone an Allied airborne assault.
Stupidity: When they are preparing for a German assault towards the end of the movie, Miller explains to them the concept of the sticky bomb and adds, "If you have a better idea of knocking the treads off a tank I'd like to hear it." Um... The bazooka? They had eight rounds for it, and it could have easily been used to disable the treads. And the men wouldn't blow themselves up with it. While it couldn't destroy a tank by penetrating the armour, it's certainly strong enough to disable the tracks.
Suggested correction: Allied forces familiar with the Panzer VI "Tiger", a 60-ton Main Battle Tank during the war knew that the armor is very tough and, even with support fire from a friendly tank, the odds of destroying a Tiger tank with a bazooka like Horvath's are pretty small. From the infantry perspective, techniques that were developed and employed in order to combat heavy Tiger tanks focused mainly on disabling the tank rather than destroying it. Anti-tank weapons of the era, such as the bazooka, were ineffective against most areas of the Tiger's armor, so specific weak points in the design were the focus. Hitting the Tiger in the tracks, suspension, engine compartment, observation slits, and in the joint between the main body and turret were some of the common weak points. Tiger tanks could only be destroyed head-on or from the sides by land mines, or direct hits by heavy artillery shells, or bombs dropped from aircraft. In the film, the first Tiger is disabled by taking out the tracks with "sticky bombs" followed by grenades thrown in the turret hatch. When Horvath fires at the second Tiger, both shots are placed on the joint between the body and the turret, the idea most likely being to hinder or incapacitate the turret's ability to swivel left or right. As the war went on, the Allies developed better strategies for disabling Tigers. One example involved British Cromwell or US Sherman tanks trying to "flank" a Tiger by working in squadrons or columns. One or more tanks would act as a diversion to keep the Tiger's crew focused in front of it while another tank would maneuver behind the Tiger and hit it in the rear section where its armor was the weakest.
Stupidity: When the Hilts discover they're 20' short of the trees, why wouldn't he "dirty up" his white/light tan pants so the tower lights wouldn't expose him so easily?
Suggested correction: Because he would look scruffy and dirty once he got out. He was the Cooler King and knew his way around after all.
I would suggest in the reality of the escape, light colored clothing would not have been worn so as to blend in to the night and the surrounding environment better. The light slacks were strictly for artistic reasons.
Stupidity: As the position of the Tiger was perpendicular to the Sherman platoon's line of march, and they were advancing across totally open country with firm ground all around, it didn't matter a hill of beans whether they shot the first or the last tank in the column.
Suggested correction: You really didn't explain the context here. Was there a big discussion of which tank to shoot first?
The mistake is saying that the tank column was travelling through an open field. It wouldn't matter if Tiger 131 hit the lead or rear tank in the column as they could manoeuvre around the disabled tank. A somewhat valid point.
You missed the point of the correction. Is it shown in the movie that they specifically targeted the first tank?
Stupidity: When Hugh O'Brian and the woman are on the beach, the Japanese planes shoot at them. Most unlikely that a plane would have gone out of formation to shoot at what appears to be a couple of civilians cavorting harmlessly on a beach, nor would the rear gunner have wasted his ammo on them.
Stupidity: Staff Sergeant Duquesne is tossing and flipping rifle grenades before affixing them to his rifle to launch against the German tanks. A battle-hardened no-nonsense sergeant like Duquesne would never waste valuable time showing off, and would definitely not flip or toss around explosive ordnance like rifle grenades.
Stupidity: In the scene where the Japanese pilots are attacking the main airfield, at one point in the background, you can see a Bofors Anti-Aircraft gun, yet nobody bothers to use it.
Suggested correction: Maybe it doesn't have any ammo, or there is nobody around who actually knows how to use it. These things can happen in a surprise attack on what is basically a holiday resort.
Stupidity: Unless Snow has solid proof like pictures or video why should Katniss believe her main enemy's word that it was Alma Coin, not him, who killed those kids?
Suggested correction: This is a question and not a stupidity. Katniss didn't just accept Snow at his word. Earlier Gale had mentioned an attack strategy to inflict maximum casualties on medics that matched the attack on the Capitol. Then Gale could not deny his part in Prim's death. All the evidence pointed to Coin being responsible as Snow suggested.
I appreciate responses here because it means you're reading stuff and that makes my day.