Apart from the USB drive, nothing actually went wrong (although as that was the main point of the experiment it was a pretty disastrous failure, overall). I later found out that the black and yellow skull icon is genuinely the default background for root on MEPIS - it does make sense to warn the user that they're logged in as root, but they could have thought of a less alarming way to do it!

What's most interesting is that I posted the same story on a newspaper blog that I started recently, and people were instantly all over it even though I'd never received a single comment before. Mention Linux anywhere on the Internet and you'll get noticed...