Fun story close to a decade ago we were attempting to upgrade our batch scheduler called Tidal to version 6x which had a RESTful API.
One of the reasons we dropped the product was because we were getting 200 status codes meanwhile the output was a java dump of an error message.
They were adamant that this was an us problem, no matter how much I tried to explain to them with numerous links explaining to them that if something has a 200 status code that should mean things worked.
They argued that the 200 meant we were hitting the API fine. We would have to write code to read the return for if it was a error or not. I still don't think they understood how stupid they were, even all these years later.