Factual error: When young Edward is hiding in the closet before his father kills himself, the child is described as having put on his father's admiral's uniform. The collar insignia, however, is an eagle - the insignia of a Navy captain.
Factual error: When Matt Damon steals the brief case and reads the memo listing the Nazi party members - the title of the memo is set in Arial which wasn't available as a font until 1982. Furthermore, the memo looks laser copied rather than typed. The fact that this is a closeup wiew makes this a rather obvious mistake.
Factual error: When Matt Damon steals the briefcase from the teacher, he reads a memo listing the Nazi recruits, the title of which is written in both English and German.The German translation is wrong. It begins with the word "The" which should be translated as "Der".
Factual error: Matt Damon's German secretary's hearing aid in 1945 could not have been that small, as the transistor wasn't even invented until 1949.
Chosen answer: It means you need to avoid getting caught in the crossfire. Originally, terms like "tying your shoe" or "pulling your socks up" meant ducking to avoid flying bullets. In a more generic sense though, it needn't be about gun violence per se. Just that you don't want to be on the receiving end of violence meant for someone else. In the movie, this was a way of telling Edward that if he could not defuse the situation, then others would be forced to harm the professor. Here, they weren't actually going to put Edward in harm's way. But if he failed to change the professor, then violence against the professor was forthcoming.