Drawn from my Band of Brothers viewing. Shibboleths appeal both to the history nerd and the word nerd in me.
During the Battle of Normandy in the Second World War, the American forces used the challenge-response codes “Flash” – “Thunder” – “Welcome”. The last response was used to identify the challenger as a native English speaker (and therefore not an enemy), whereas the German enemy would pronounce it as “Velcome”.
Similarly during Operation Chariot the British raiders used the challenge “War Weapons Week” and the countersign “Welmouth”, likewise unpronounceable by most Germans.
The British also used the word “squirrel” as a shibboleth against the Germans, who would frequently pronounce it “Sqvirrel”
In the Pacific Theatre of Operations, the shibboleth was lollapalooza, whose pronunciation produces severe difficulties for native speakers of the Japanese language
Woolloomooloo was used by Australian soldiers in the Pacific Theatre during the Second World War to identify themselves when approaching a camp.