An array has an order. BigQuery's OFFSET counts positions from zero, while ORDINAL counts from one. The first item is items[OFFSET(0)] or items[ORDINAL(1)]. Reading a position that doesn't exist raises an error with the ordinary forms. Use SAFE_OFFSET or SAFE_ORDINAL when a short or empty basket is valid; a missing position returns NULL.
UNNEST expands an array into rows but doesn't preserve row order by itself. Add WITH OFFSET AS position to capture each element's original zero-based position, then sort explicitly when the output must follow that order.
The fresh basket fixture includes empty arrays. They make a useful test: an inner expansion drops their parent orders, while safe indexed access keeps the order row and returns NULL for the absent item. Decide which behavior the report needs.