Your OpenAPI file marks shipping_address as required on GET /orders/{id}. After Tuesday’s deploy, about one response in 300 leaves it out, because a new code path for orders created by the import job skips the field. The docs still say required. The tests still pass, since nobody wrote one for import-job orders. A client with a strict deserializer starts throwing on 0.3% of order pages, and the first report you get is “sometimes the order screen is blank”.

An API’s documentation lists the fields a record can have. It rarely tells you how many records actually have them. Those are different numbers, and on open data APIs the gap is often enormous.

The failure looks like this. You read the schema, spot a field that would make a great feature, build the feature, and then discover the field is populated on four percent of the data. By then the feature is written and the pitch has been made.