While uploading a screenshot for a post on Super User, I managed to get the bizarre response Request returned an error: [error] I'm a teapot.
In balpha's answer to Stack Overflow returning HTTP error code 418 (I'm a teapot)?, he explains that this error code is used for CSRF violations.
A while ago we finally switched the response to CSRF violations from a non-descript error page, as we had been doing for years, to something sensible that somewhat explains what's going on.
While implementing this change, I was considering what HTTP status code to return in such a case...
Long story short: There's not really a good status code for this case, so I figured we might as well return a 418. In addition, it's a little easter egg you just found :)
At the time I was uploading the image from a work PC running Internet Explorer 10*. I only encountered the error once, as after refreshing the page it did not occur again.
Why would Stack Exchange's image uploader return a 418 teapot error?
*Yes, I know IE10 is outdated and insecure. I'm not using it by choice.