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Your ledger says one thing, the processor's report says another, the bank says a third. Until those three agree you don't know what happened, you know what your code thinks happened. Three-way reconciliation is how you find the gap before a customer does.

The payment that hurts isn't the one that fails loudly. It's the one that succeeds twice. Idempotency keys, webhook discipline, and a charge modeled as a state machine are what stop a retry or a redelivered event from charging a customer twice.

Most fintech products don't fail on the features. They fail on the money math, and the money math was never what the demo showed. A builder's map to what decides it: idempotent payments, an immutable ledger, reconciliation that ties out, and the moat that compounds by 2028.