Delivered at the QCon London conference in 2009, this talk by Tony Hoare revisits a decision he made in 1965, while designing the type system for the language ALGOL W. He had introduced the null reference, a way of indicating that a variable pointed to no object.

Hoare explains that he added the feature simply because it was easy to implement, despite the risk that any attempt to use such a reference would fail. In the decades since, null references have been a persistent source of errors, crashes and security weaknesses across countless systems. He estimates the cumulative cost at perhaps a billion dollars, the phrase that gives the talk its name.

Presented partly as a public admission, the talk uses the example to reflect on the lasting consequences of early design choices, and on the value of language features that prevent such errors by construction.