Stupidity: A prisoner, Jack Williams, rats out Hogan and his team to the Germans, so they fake his death and smuggle him back to London for court-martial. What a stupid thing to do. They should just shoot him and be done with it. First, what if he escaped from captivity on the way through occupied Europe? Second, he has done nothing wrong. Being a grass isn't a crime. Third, Hogan and his men are the war criminals - they often engage in combat activities out of uniform. Williams is going to be acquitted.
Stupidity: When an officer is speaking to who he thinks is colonel Tuttle but is really Radar with a surgical mask, he says what he needs to and is about to leave. Radar who was alone at the time and was completely in on the fact that Tuttle never existed, could have kept up the façade and let him leave, but instead chooses to reveal himself which was utterly pointless.
Stupidity: Adam, Jason's replacement on Bravo team is operating as Bravo 1. He's in the midst of reloading his carbine rifle with a fresh magazine when he sees a suicide bomber running towards his team so instead of drawing his pistol which takes 0.7-0.8 seconds; he's a trained professional so that timing is reasonable; he instead charges the suicide bomber and dies in the explosion. It would've taken less time to shoot the threat than to charge him. (00:07:00 - 00:08:00)
Stupidity: When Eugene approaches Tsuda after Tsuda is shot by Ae-sin, Tsuda attempts to shoot him, but after pulling the trigger, he realises there are no bullets in the gun. Seconds later, a group of soldiers run up to him, and he attempts to shoot them, but he can't because the gun is still not loaded. Something he should have known since he only just attempted to shoot Eugene and failed because of it. (00:50:00 - 00:50:30)