Introduced on day 7
Join fan-out
Row multiplication that happens when the right side of a join has several rows per join key, so measures from the left side are repeated and their sums inflate.
In detail
Joining flight results (one row per flight-date) to a booking curve with 13 snapshots per flight-date returns 13 copies of each passenger count. Detect it by checking key uniqueness on the right side and comparing row counts before and after the join; fix it by filtering or aggregating the right side to one row per key.