Adage invoked in the talk: the way a system actually works is the specification, regardless of what the documentation says. As soon as V1 has real users, V2's 'correctness' is no longer the judge — any observable behavior someone depends on becomes part of the contract. Wójtowicz brings it up while discussing the stuck-on-V1 customer and why the decision to release V1 should already consider whether you can live with it forever.