click Login button from http://en.wikipedia.beta.wmflabs.org
Login as usual
Get red-box error "Login error
There was an unexpected error logging in. Please try again. If the problem persists, it may be because you have cookies disabled, and you should check that they are enabled in your browser settings. "
No records appear in the fatals log when this happens.
After a brief inspection, I see this in both automated tests and manual operation, and it happens more frequently manually.