Niven in WWII, specifically the Battle of the Bulge.

As Germany tried to turn the tide in the last few months of the war, Hitler sent specially trained commandos into the Allied lines. The commandos were to impersonate Allied troops and sabotage Allied plans as best they could. American troops learned of the ruse and implemented passwords and recognition questions that only those deeply steeped in American culture could readily answer. ( For whom did Babe Ruth play baseball, for instance.)

By this time David Niven was serving in the British Army and was stopped by American sentries who did not initially recognize him. When Niven did not know the answer to the password question put to him, the situation escalated. Finally, Niven exclaimed something like, " I do not know your password but a few years ago I did a movie with [insert name off famous American starlet]."

The sentries then realized who they had, and they released Niven.

I'm not certain what film Niven referenced, nor the starlet, but I think it was Bachelor Mother and Ginger Rogers that saved his skin.