Side effects and non-determinism can break the relationship between action ("command") and event. You'd need to take care with things such as random number gene

Side effects and non-determinism can break the relationship between action ("command") and event. You'd need to take care with things such as random number generation, to use a PRNG whose call sequence is determined only by the actions, and whose seeding is an action itself. Side effects caused by action handlers are also something to beware - don't launch the missiles when you're replaying the past.