As to the lock up crash, I think that it's just a rare-enough race condition
that you wouldn't expect to see it except once in (quite a lot of traffic)
times. It's a bad one, though: the screen goes black and the PC stops
responding, as if it had dropped into a standby or hibernate state. Except
that it doesn't respond to anything you do to try and wake it, including
hitting the soft power button, pressing keys or moving the mouse, and plugging
or unplugging USB devices (which sometimes helps flush through the system when
its gone wrong by triggering interrupts and other IO activity).