This might just be artifacts of staring at a screen too much, but:

• I've seen more than one web page with a background that had some bit that looked like a hair or speck of dirt sitting on the screen. And reflexively tried to wipe it off the screen.
• My old laptop had dead pixels that I would, every now and then, think were dust specks on the screen and try to remove.
• Just noticed a strange rounded and transparent little button pop up on a web page, with some odd icon inside, so I clicked on it. It seemed to react to the mouse but nothing else happened. It turned out to be a water droplet, magnifying what was behind it in confusing ways. (They don't call it "Aqua" for nothing..)
• Must upgrade my PowerManga fighter to kill the attacking moths. Die die die!

Hmm, it may be time to write my second ever X program. Exporting random "dead" pixels to an open $DISPLAY would be so Evil.