Clinic slots on Postgres. Fire concurrent bookings at the last seat and watch exactly one win.
Both paths run against the same table. The unique index on (slot_id, seat_no) is the guarantee —
the read-modify-write path computes its seat number from a stale count, so racers collide on it and the database refuses them.
A CHECK constraint could not do this: a CHECK sees one row, not the count of its siblings.