Hillary Clinton, the Democratic presidential front-runner, approached a Bronx subway turnstile on Thursday with the casualness of a regular commuter, MetroCard in hand.

“Moving, moving,” someone called out, perhaps from her security detail, as she began to swipe the card.

But by that point, all movement had stopped: The card would not comply. It would take Mrs. Clinton five swipes before the desired beep-click of success finally came. And with each swipe, she, like others before her, demonstrated the potential perils that await politicians in the subway.

There are other routines of New York life that have the ability to similarly trip up politicians. Indeed, two of the remaining three Republican presidential candidates — John Kasich and Donald J. Trump — have been seen eating pizza with a knife and fork. (“Frankly, it was very comfortable,” Mr. Trump, a Queens native, said in response to local outcry after a campaign appearance in 2011 with Sarah Palin in which neither used their hands. “I like not to eat the crust.”) Even the city’s mayor, Bill de Blasio, has tripped over the third rail of local pizza consumption.