How it happens
An agent (or a developer being tidy) standardises casing or naming conventions. `checkout_completed` becomes `checkoutCompleted`, or `order_placed`, or vanishes into a constant in another file. Each is a legal change. None of them updates the dashboards downstream.
What breaks downstream
Old data uses the old name; new data uses the new name. The funnel splits in two. Time-window metrics show artificial dips and spikes during the transition. Recovering means UNION-ing two event names in every affected query.