Let's say you have an UPDATE query which uses old values to calculate new ones. The best example for now is swapping two columns:
UPDATE table SET f1 = f2, f2 = f1. There is one SQL engine as far as I am aware where this breaks: MySQL (it works on PostgreSQL, SQLite and MS SQL). On MySQL, f1 and f2 both will have the old value of f2 instead of swapping.