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.