(note: Please read entire post before replying. Ta )
Every time I plug a certain USB memory stick into my Raspi, it shuts down!
This is not a problem with the Pi, it is a deliberate effect.
I often use my Pi as a simple media player. No keyboard or mouse, just a few simple udev rules that run on insertion of certain USB memory sticks.
Any USB device that has a first partition having a label of "VIDEO_nnn" will be automounted and a menu run that allows playing of video files recorded from my MythTV system on it.
One particular memory stick (an old 128MB, unreliable unit), with a certain UUID will trigger "shutdown -h now".
It's a very handy way of doing a controlled shutdown of my Pi when I have no other way of connecting to it