Examples of shibboleths in movies:
In Cannibal! the Musical, the group of prospectors encounter a Confederate soldier. They attempt to convince him they are fellow southerners in order to avoid his wrath, but give themselves away during a sing-a-long, when one member of their party ends a verse of "Dixie" with "You stupid Yank!" Though honestly, I'm not sure why that isn't an acceptable alternative to the real words ("Dixie Land")!
In Inglorious Basterds, [Spoiler (click to open)]a British spy posing as a Nazi gives himself away when he orders three drinks by holding up three fingers rather than a thumb and two forefingers, as is the German way. Actually this one contains quite a few shibboleths! Also, corpses.
Shibboleth comes from the Hebrew word shibbólet, meaning the part of a plant that contains grain, such as a stalk of wheat, or a rushing, flooded stream. It acquired the meaning explained above as a result of it having been used, according to an account in chapter 12 of the biblical Book of Judges, by Gileadites to distinguish their own from Ephraimite fugitives following a military victory. Anyone wishing to cross the Jordan River was asked if they were an Ephraimite, and, upon their (very wisely!) answering in the negative, asked to say "shibboleth." As the Ephraimite dialect was apparently lacking in the sort of sh sound found in words like "shoe" and "shudder," Ephraimites invariably sealed their fate by mispronouncing it as "sibboleth."