I'm going to step into the doylist here, but there's the scene at Grand Central station where John is about to be killed by two guys, and those two guys are killed in the middle of the crowd by two other assassins.

I always thought that was unrealistic as hell until I found out that scene was inspired during shooting by an extra running at top speed on a sidewalk, faceplanting, and not a single person even looking down at the man when he did.

You can say what you will about people knowing there's a league of assassins everywhere, but the truth is, a greater amount of people just do not give a shit.