There's must be an energy leak somewhere in the system. Instead of it's simulated perpetual motion, the bob eventually increases speed so that it ends up going right around a full circle, above the top of the java applet. It's happened twice in a row now as I've left the applet running in the background while going through physics questions. Try it out yourself if you're interested. Leave the applet running for at least an hour, and it ought to go wild.