Comment by Izkata

It can happen out of necessity: if the failure is in an ajax request and you need to send back a message or additional data in json, apache eats the body of error responses. So a success response is all that's guaranteed to get through.

I don't know about others, I know about this one because I had to dig into a bug where something on live looked like it succeeded but didn't, while the error worked fine on dev. Ended up downloading the apache source and finding where it was happening before just using a 200 response.