When trying to log in with cookies disabled, and entering the wrong username or password, the login prompt will say “Unknown error”.
Yes, it makes no sense, to try to log in with cookies disabled, but it can happen by mistake; the error message is remarkably unhelpful. (It looks like some kind of CSRF failure, so probably it should just say that.
I too believe that error messages should be more clear and point towards a solution. Moreover, the fix is pretty easy and I believe some users will love it!
Here is the pull request I made. This is what it looks after: