Once upon a time we had to integrate our backend with the billing service of another company. They sent us a Swagger API Description, describing the payload they will send to us. Everything was ok, and we implemented it.

We told them our background was ready to run some tests, but they got some error. After some quick debug i asked for the actual payload they just sent to me.

The payload had different field types, different field names and non-nullable keys was explicitly coming as null.

THEY DIDN'T FOLLOW THEIR OWN SPECIFICATIONS!