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.
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.