My father-in-law once told me about himself and a comrade of his. They were on some sort of perimeter duty/patrol on the eastern front (he was in a German unit). It was pitch black out and they were trying to move quietly and cautiously when his friend stepped into a hole and fell awkwardly. His friend let out a stream of swear words. Now depite being in a German unit, his friend was bilingual, speaking both German and Russian. He swearing was all in Russian...
And out of the dark, about 10 meters away, a Russian voice in dark told them to be careful and be quiet!
Needless to say, they shut up fast and as calmly and quietly as they could headed back to their own lines! He ended his story by saying that the Russian position they'd stumbled on was later dealt with while it was still dark.
So, now and again it happens. Soldiers will speak out even when they meant to be quiet!
