Be careful about your pronunciation.

The follow excerpt is from The Ethnic Phenomenon, by Pierre L. Van Den Berghe. The context is the use of language as an ethnic marker.

"Although language is a relatively subtle test, certain easily detectable
phonemes (音位) give the foreigner away. A classical historical episode concerns the massacre of French troops by Flemings in the city of Brugge (or Bruges as the French say) in 1302. The Flemings wanted to kill the French army of occupation in their beds without raising the alarm, so the problem was how to identify Frenchmen quickly and reliably, in the dark, in order to slaughter them without fuss. The solution was to make them repeat a short Flemish phrase, "schilde en de vriend," ("shield and friend") that contained phonemes unpronounceable to native speakers of French. History books tell us that the stratagem was devastatingly successful."