When the minecraft authority servers (I guess, might be other servers) are having problems, people are unable to connect to 1.6.4 multiplayer servers. See https://mojang.atlassian.net/browse/MC-40683 .

Expected behaviour: Client should report that mojang's server XYZ is timing out, overloaded, or similar. User expects more accurate error messages to avoid them wasting time reinstalling, or changing passwords, restarting routers, disabling firewalls, when none of those actions will resolve the problem.

Actual behaviour: Client reports Bad Login