At work I tried the XP background that looks like the LCD cracked, and a picture of IE with ALL the spammy toolbars installed (such as Bonzi Buddy)

Infact Sysinternals used to have a screen saver scr file with a selection of BSODs from various operating systems

xscreensaver still has this. I have it on my laptop. Sometimes it triggers when I'm out consulting. Clients think the machine crashed. How could you not notice this is a crash screen from HPUX?

I believe this (or a descendant) still exists in Fedora, and it got my fellow Linux user coworker to double take once :)