Build event-sourcing tests - the given-events / when-command / then-events aggregate test, replay determinism (same events produce the same state), event-versioning + upcasting, snapshot equivalence (replay-to-N vs snapshot-at-N must agree), projection rebuild from the event log, and retroactive event correction. Per martinfowler.com EventSourcing reference. Use when an event-sourced aggregate gains a new event type or a changed payload schema, when snapshots are introduced to shorten replay, or when the event log is the audit system of record.