First, this is just for fun, and has no practical use at all. Second, see the warning below before trying this...
Everyone knows the old fairground trick of infinite mirrors, where opposing mirrors give an image repeating itself into infinity. This was also replicated famously in Queen's Bohemian Rhapsody video. Well, try this...from Mac #1, connect to Mac #2 using screen sharing in the Finder (not iChat). From within the screen sharing window, connect Mac #2 to Mac #1, again using screen sharing. Stand back and wonder what you can use this for!
Warning: I had trouble getting out of this mode once in it. To do so, I put Mac #1 into sleep mode (press and hold the power button), and as soon as I woke it again, was able to quit screen sharing. YVMV, and I have no idea what effect different window sizings or options may have. Fun for the child within though :)
In reality I think this is actually a bug, or at least an oversight by Apple. With my sensible head on, the app should warn you and/or make this impossible.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071118083642465