New Delhi, Sept. 24 -- This is a fishy tale with a long tail. I visited Aurobindo, my usual fish vendor in Chittaranjan Park, New Delhi, two weeks ago to purchase my stock for the week. I left after paying the bill of Rs.2,010 through my cellphone, or so I thought, marvelling at the convenience of India's Unified Payments Interface (UPI), one of the most advanced public digital infrastructure systems in the world.

A week ago, Aurobindo called to say that I had paid him Rs.20,010 instead of Rs.2,010 by mistake, wanting to return the Rs.18,000 that he now owed me. That evening, I visited him to collect the balance, leaving a generous gratuity for him and his colleagues, which he reluctantly accepted.

Embedded in that little tale are two i...