What operational risks should you consider before triggering a full event replay on a production system?

The two main risks are load and staleness. Replaying a projection's full history means the event store has to serve every event from the beginning, on top of whatever normal live traffic it's already handling — for a large event store, that's a meaningful, sustained read load that can affect other consumers. Meanwhile, the read model being rebuilt is temporarily incomplete or stale for the duration of the replay, so anything querying it during that window may see partial or outdated results. The practical mitigation is to treat a replay as a planned operation rather than a casual one: run it during a low-traffic window, monitor event store load while it's in progress, and communicate to anything depending on that projection that results may be temporarily stale. For a very large event history, it's also worth estimating how long the replay will actually take before triggering it against a live production system.