Write Skew: The Phantom Anomaly of Snapshot Isolation

Write skew is a database anomaly where two concurrent transactions read the same state, make decisions based on it, and then modify different data items, unknowingly violating a business rule. It's like two people seeing one spot left in a room and both letting someone in. This occurs under Snapshot Isolation when enforcing constraints across multiple rows, like in booking systems. The main footgun is assuming Snapshot Isolation is truly 'isolated' or serializable; it allows these logical conflicts that simple row locking misses.