Ran into this bug in Beta mode (on beta labs), but may also exist in stable.
I made an edit to an article, went to the preview screen, and then hit the button to save the edit. The GET request to the API to retrieve an edit token returned a 503 error (i.e. a Wikimedia error, not an API error). This left me in the "saving edit" state forever, with no way to escape other than hitting the back button and losing my edit.