Learn how to go deep undercover in a hostile foreign country

Undercover is a World War II training film from the OSS, precursor to the CIA, would be enough to dissuade most people from a career in espionage. They enact numerous examples of tiny slip-ups that ended up blowing the covers of various spies and secret agents.

These training films are almost always excellent, because many were produced by pros from the Hollywood studio system. The examples are reminiscent of the nerve-racking pub scene in Inglourious Basterds, where a tiny slip-up while ordering drinks leads to blown cover and subsequent carnage:

Undercover (h/t Muckrock, which just did a piece on the OSS WWII "morale operations" field manual on bribery and blackmail)