Get too close to this cog and gear-driven peep hole, spotted in action at Maker Faire, and you could lose more than just an eye. Hence, the thoughtful "No Finger Zone" signage that greets operators when they turn the crank.
You can see the sign and the peephole in action in the Maker Faire slideshow below, or you can head over to this link for just the video.
Want to make one yourself? Easy. Just head over to the ShopBot forums, where there's apparently all the DXF files you'll need to mill one of your own.
Update: If you can't fine the "zone" in the source links, here's a screen grab: