AP Photo

Early in my time in China, I learned a useful lesson for daily life. In the summer of 2006, I saw a contingent of light-green-shirted People’s Liberation Army soldiers marching in formation down a sidewalk on Fuxing Lu in Shanghai, near the U.S. and Iranian consulates. They looked so crisp under the leafy plane trees of the city’s old colonial district that I pulled out a camera to take a picture of them—and, after pushing the button, had to spend the next 60 seconds running at full tilt away from the group’s leader, who pursued me yelling in English “Stop! No photo! Must stop!” Fortunately he gave up after scaring me off.

The practical lesson was to not point a camera toward uniformed groups of soldiers or police. The broader hint I took was to be more careful when...