I think I've figured out why
http:200 ERROR
Is sometimes returned from some APIs... Sort of legit

Does it have a success flag set to false?

@asgs yeah.

Some NW infras/WAFs might have global rules to return an ERROR html if a webapp returns anything but 200. Especially in old, legacy infras where all the content used to be html-only. So until all the webapps are taught to handle errors correctly infra might not want to lift the non-200-err-page.html rule. Until then you still have to deliver and the success flag is a viable workaround along with the always-200 http code

The other 2XXs result in an error page, too? 😐

@asgs I think yes. Didnt try them tho

It's because people don't know wtf they're doing.