German parachutists who were caught in American uniform during the Ardennes Offensive were considered under the rules of war to be enemy "spies" and were executed.
Note that international law does permit the use of enemy uniforms as a legal "ruse de guerre", but clearly states that you are only allowed to use enemy uniform up to the point of contact. You are not allowed to fight while in the enemy's uniform.
Germany's Brandenburg commandos, for example, used enemy uniforms on some of their missions. The Allies made limited use of this too; in the spring of 1945, a company of American soldiers approached one of the Rhine river bridges wearing German helmets and overcoats in an attempt to take the bridge, but their ruse was discovered and the bridge quickly blown up before they could storm it.
In other words, these things didn't just happen in the movies.
The German parachutists probably didn't do as much damage as the fellows in that 1960s movie, but that wasn't the point; whatever activities they were conducting, they had not changed back into their own uniforms and therefore, my understanding is that they were found guilty - not sure what the exact term would have been; "espionage" perhaps. There are pictures extant of the executions - it was done by firing squad.