A developer's payment system processed 47 duplicate charges, costing $2.3 million in chargebacks. The root cause was a flawed PostgreSQL schema using `DECIMAL(10,2)` for money, which silently rounds microtransactions and invites race conditions. This incident exposed critical flaws in handling financial data.

Floating-point arithmetic is unreliable for currency. The solution is storing amounts as integers in the smallest unit, like cents, using `BIGINT`. This prevents precision errors. Additionally, idempotency keys are mandatory to prevent duplicate charges from client retries, a common user behavior that breaks naive implementations.

Beyond precision, robust systems require row-level locking with `SELECT FOR UPDATE` to prevent concurrent overdrafts. An immutable ledger via an event-sourcing pattern is essential for audit trails, allowing reconstruction of any account state. The author provides a complete, production-ready schema and stored function to handle these edge cases.