Stupidity: Like in a previous movie adaptation (and unlike the novel), Lombard finds out way ahead of time that only another 'Indian' is left besides him, and knows they have the gun, but he lead them to the last body turning their back and leaving the gun to them, against any logic and declared purpose. It makes even less sense in this version than in the 1945 version, given the outcome. In the novel, the gun is stolen from him.
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: 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.