The "teapot" error is one of the various responses we return if something looks very suspicious at the http/form level - most likely because of an XSRF check failure. On mathoverflow, this is usually from bots that spend their entire life trying to edit one post (seriously, it is always the same post - I have no idea why, there's nothing particularly unusual about this one post). But yes, I can see that you got that error 3 times on the 28th.
(edit: removed a paragraph relating to profile images, I went a bit mad there)
I am investigating. It looks like it is generally working; I've tried to repeat your steps using the same browser as you (slightly different OS), and it worked fine.
Update: I have identified a scenario that could theoretically cause this result and which would be consistent with your recorded activity. Could you please try to indicate exactly which options you used on the image upload? Did you:
- paste an image
- click and select an image from your file system
- drag and drop (and what: a file? an image?)
- provide a url
OK, I'm 98.2% sure of what happened, have a working repro that causes the problem locally, and am in the process of pushing a fix. Basically, the XSRF protection for the image upload form would not be updated in the scenario:
- start writing / editing a post
- show the image upload form, close it
- do "stuff" on other pages - searching, etc (you searched, according to our http logs; searching is good - I applaud you) that causes your XSRF token to be rotated
- reopen the image upload form, and do something that causes the non-ajax submit to be used (basically, anything that doesn't have a preview)
- attempt to submit it
In that rather convoluted scenario (which matches exactly what I can see from your logs), the old XSRF token (the one from when you first showed the image upload form) would be used, and it would be rejected. However, when you get to submit the actual post, the correct XSRF token will have been used (via some evil browser magic).
As I say: I'm almost certain that this is what bit you, and a fix will be deployed today. Note that rotating XSRF tokens is a rare event, so you might also want to think about purchasing a lottery ticket for managing to achieve such an unlikely sequence of events. And then maybe reconsider it because you presumably (as a user here) have a working knowledge of how probability works.