Earlier today, I attempted to login as Voidwalker on testwiki (publictestwiki.com), but encountered an interesting error: the centralauth session cookies never got set, no matter how many times I tried to login. Furthermore, attempting to login on meta would either cause "No active login attempt is in progress for your session." or result in the same issue as testwiki. Attempting to clear cookies in the browser had no effect, as did changing browsers entirely. Eventually, by logging in, and immediately clicking logout, I was able to clear all session data, and was then able to properly initiate a login on meta that worked.
Further experimenting with the issue resulted in being able to login on testwiki, but all other wikis not saving login cookies. I attempted loginwiki, and that only resulted in the "fake" login (that immediately loses the cookie). All attempts to login on meta resulted in "No active login" errors. Attempting to clear all browser cookies had no effect, except to deprive me access to testwiki (which had, until then, been keeping me logged in). This also appears to have made the "No active login" issue propagate to loginwiki. In this case, attempting to logout on testwiki failed to fix the issue.