American soldier poses with captured German weaponry, 1944-45

Every weapon has a signature, and using an enemy weapon can invite friendly fire. Prior to D-Day, American paratroops conducted mechanical training on German weapons (there was insufficient ammo for live fire training). With many men separated from their leg bags and weapons thanks to high-speed drops (which led to violent parachute openings), picking up a German weapon was fairly common. In the well-documented E company of the 2/506th PIR, Lt. Dick Winters briefly carried a German Mauser until he could recover an M1 from an American casualty (this is described in Steven Ambrose’s book and depicted in the TV miniseries). That had no consequences for him, but Sergeant Bill Guarnere had problems when he picked up an MG42. In his own words:

I went looking for a gun, and found a Thompson submachine gun. I also took a German MG-42 off a dead Kraut and started shooting it, but the gun made a noise that was distinctly German. The German guns went brrrrrrrrrrt! The American guns went bap-bap-bap-bap-bap. Every time I started shooting it, the Americans started shooting at me! I got shot at by a dozen or so of our own men. I threw it the hell away. You learn fast or you get killed. I grabbed an M1 instead.