First a little background:
I recently wrote a reverse proxy which I have found to work wonderfully so far, and being an avid Stack Overflow user, I thought I would try it out there.
I was just playing around and everything was working perfectly, until I hit one page that suddenly returned HTTP error code 418 (I'm a teapot) which was an april fools joke from Hyper Text Coffee Pot Control Protocol.
Why would Stack Exchange ever be set up to return this status code?
I do not even know what to call this. I would at first have thought "Bug", but it is obviously not, because, well - how would you end up with an HTTP server returning 418 without doing it on purpose.
I will not dispute that this may be a result of my reverse proxy not doing something exactly the same as a browser, and because that is an OAuth authentication page and there may be some security or something, but perhaps it should be returning a more appropriate HTTP response?
As requested, more information about the post request my client is sending. It is, for instance, sending a standard HttpWebRequest pointed at https://stackoverflow.com/users/authenticate with the client's cookies/useragent forwarded, method set to POST and post values of the following: