A newspaper in New York has published the names and addresses of gun owners in Rockland and Westchester counties. Two groups of people are upset about this: the gun owners, who fear anti-gun violence (such a thing is not an absurdist term, but should be), and the gun non-owners, who fear they will be singled out by criminals.
That's the thing about positive externalities: their existence means the item in question will be under-provided. The higher the chance I break into the house of a gun owner, the less likely I am to break into any houses at all. This is the very argument that the anti-gun crowd says is hogwash, and yet it is non-owners making its case now in New York. Theory is great until it's actually your family's safety undermined by theory.