I've had no kernel panics on 10.9.3 ... but this Haswell rMBP 13" periodically froze after password entry when screen locked. And froze so badly that switching

I've had no kernel panics on 10.9.3 ... but this Haswell rMBP 13" periodically froze after password entry when screen locked. And froze so badly that switching user was no way out: I had to hard-reboot. Since disabling the screen lock on inactivity it's been okay, but that's not a terribly secure or reassuring solution.