The sign on the Upper East Side street looks so realistic, people stop and lean closer.

“Broken Smartphone?” it reads. Beside the lettering is a blown-up version of an iPhone, several feet tall, with long, spider web cracks spreading across its screen.

Smaller letters on the sign read, “Warning!” but people ignore that and reach out to touch the screen.

The glass, it turns out, is real, and so are the jagged cracks. Evidence of a petty crime that, with some luck and quick thinking, became an unintentional display of guerrilla marketing.

The luck resided in the location of the sign. It is not a window to a barber shop or a dry cleaner or a bodega, where broken glass is broken glass, something to be replaced quickly. It is an iFix store, a shop that repairs broken electronics, mainly ones that begin with a lowercase “i.”