There is a tendency for Macs to crash when disconnecting the USB cable between the Mac and a Sony Ericsson K750i phone.
This free patch fixes it; it's called K750iGrabber.
Here's a quote from the above site about the cause of the problem:
After some more investigation, it turned out that the K750i not only provides a card reader interface over USB, but also provides several interfaces for modem and device configuration functionality. As Bluetooth is plenty fast for the former, and there are no Mac device drivers for the latter, I built a simple driver that seizes all those unused USB interfaces and disables them.[robg adds: I haven't tested this one, but it sounds useful if you've got a K750i...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=200508070558554