For linux look at xroaches for a good example. Had a friend who used emacs, mostly in a big single window. He used a glossy/reflective screen. He moved windows 2-3 times and then madly looked around his desk trying to figure what was running around.

Finally figured it out and I just about fell out of my chair laughing.

Gnome has (or had anyways) an easter egg, hit alt-f2 and type "free the fish". You could click on them to get them to go away.... for awhile.