THE SLOBBERING MUTT roared up out of nowhere, barking smack, and the lab at the end of my leash leapt back in fear. So did my daughter. The daughter I fleetingly considered leaping behind.

We were in Interlaken Park, which is not an off-leash area. So being dutiful dog-sitters, we had leashed my sister’s lab. The other owner hadn’t. After a moment, up trotted the owner, identifiable by that pretend-chagrined expression common to certain breeds of pet owner and parent. Oh, will you look at what that little rascal is up to now!

“Oh, say!” I managed with as much pretend-calm as I could summon, while laboring to keep our dog from fleeing the state. “You know, this is an on-leash area!” Upbeat! Positive! The owner’s carefree expression went dark. “Well,” she sputtered, “it’s people like you who keep dogs on leashes that are the problem!”

Um… what? Clearly my comment triggered hostility. But dementia? My passive, leashed, nearly eviscerated dog was the problem? As we walked away I got madder with every step, especially since I was rapidly gathering that everybody walks their dog off-leash in Interlaken Park. (At least they did before the animal enforcement people read this.) Did that put me in the wrong? Was I just the newbie who wasn’t wise to the ways of the neighborhood?

Of course not, assured my husband later. When someone’s illegal behavior threatens your security, as Cujo and his delusional human did, you have to speak up. Simple as that.

Playing social vigilante almost always exacts
 a higher price than enduring the social offense.

Yet not simple at all. Because sharing a close urban environment with strangers raises versions of this issue all the time. What about the coffee-break smokers who huddle in a downpour under the overhang that is no way 25 feet from an entrance? Technically, they’re breaking the law. Technically, their secondhand smoke is doing me harm. No, I’m not thrilled about it.

But here’s the pathetic truth: Playing social vigilante almost always exacts a higher price than enduring the social offense. I’m thinking of the sketchy character sitting across from me on the bus one day last month, who peppered his thundering conversation with never-before-heard conjugations of the F word. As I sat rapt at the sheer inventiveness of his usage— could he be a linguist? —I grew steadily more uncomfortable. It was a crowded bus, and unnaturally quiet. I noticed a couple of kids. Little kids. Okay…so now this is getting kind of awful. But say something? Um, no.

Suddenly from up near the front came a voice: “CAN YOU CUT THE FOUL LANGUAGE PLEASE? THERE ARE CHILDREN IN HERE.” It rang through the bus like the clarion of Winged Victory herself; a mighty female in full take-no-prisoners bellow. I cringed in anticipation of the linguist’s retaliation. Then, in chastened squeak: “Uh…sorry, ma’am. Won’t let it happen again.”

I’m still puzzling over whether his humble acquiescence was triggered by her invocation of children. (Impressive strategy, ma’am.) Would he have been so contrite if she’d asked him to stop the foul language, period? Because then her chastisement, of behavior that was not after all illegal nor physically harmful—no matter how welcome by 99.9 percent of the bus—would’ve smacked of self-righteousness.