The odd thing is that PS/2 uses open collector signalling - the data lines are pulled up to +5V by a resistor, and to transmit data the lines are pulled down to 0V by a transistor. This is inherently very safe, it doesn't matter (electrically) if both ends try to transmit at the same time. Hot plugging *should* be safe. But it causes problems because of power surges. Why this doesn't happen every time you power on with a keyboard connected I've never quite worked out. Maybe normal power ups have the 5V power rising smoothly, more by accident than design?