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".
Continuity mistake: On Deer Island during the dinner party before the two lead characters marry, Angelina's character is seen to be carrying a glass of white wine. The scene changes, and the wine is now red when she sets it down on the table.
Other mistake: During the 25 years time span of the movie, all of the characters age noticeably except for Clover and Damon. He, miraculously, goes through his teens, 20s and up to his late 40s gaining just a couple of gray hairs and a few wrinkles around his mouth, plus the same young voice from the early days.
Plot hole: Near the film's end, when Edward Sr.'s son, Edward Jr., is about to be married, Edward Sr. approaches his son looking very grim. He is about to tell his son that his fiancee has died. At this key moment, Edward's wife exclaims to her husband, "What have you done?!" Logically, this does not make sense, since Edward's wife, Margaret, has no knowledge (as far as the audience knows) of the Bay of Pigs leak made by her son, nor of her son's fiancee's spy role in obtaining the information. Without knowing this information, Margaret would have no reason to suspect that her husband was involved in some bad incident involving her son's fiancee.
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.
Continuity mistake: Matt Damon receives a tape in the mail. He listens to it, rewinds, listens again, then fast-forwards it. But instead of getting the fast-forward bit, we get where he left off.
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.