Test async ordering - event-loop / queue / channel ordering assertions, JS Promise microtask vs macrotask ordering, Python `asyncio.gather` vs `asyncio.wait_for` semantics, Go goroutine + channel happens-before relationships, async/await re-entrancy. Use deterministic schedulers (sinon fake timers, asyncio test mode) to remove run-to-run variance. Use when a callback fires twice, a later response overwrites an earlier one, or a cancelled parent task leaves a child still running - bugs where completion order, not shared memory, is the defect.