This 1944 OSS spy training film about undercover work implies that small details, such as the way one ties a knot when tying down a boat, can blow a spy's cover. Is this true, and are there any examples of World War II spies having their cover blown this way?

by condax

Here's the film in question, titled Undercover.

Other examples include a Spanish spy in America being confused about the slang use of the word "killer," and a British spy in Germany using a particular hair gel that was only available in England.

So I'm curious: is this paranoia on part of the OSS? Or do mundane details such as these really blow a spy's cover? If so, are there any examples?

hdaviirus

This is kind of related, but in Inglorious Bastards, one of the allied spies in the movie blew their cover when they displayed the number three on their hand in a "non-German" way. Do different geographical regions represent numerals on their hands in different ways, and is Tarantino's plot point based in reality off some kind of real event, or was it just artistic license?