The foreign key that documented an assumption

A constraint added not for integrity, which was already sound, but because the relationship was undocumented anywhere else.

ALTER TABLE invoice_lines
  ADD CONSTRAINT fk_invoice_lines_order_line
  FOREIGN KEY (order_line_id) REFERENCES order_lines(id);

-- the data was already consistent. what the constraint
-- adds is a statement, in the schema, that an invoice
-- line corresponds to exactly one order line — which
-- three people had assumed and one had not.

A schema is the most-read document in an application and the only one that cannot go stale, which makes a constraint a good place to put a fact. The cost is a delete that now fails loudly where it previously left debris, and that behaviour change is worth announcing rather than discovering.