It should go without saying that you should follow all posted rules while in the National Parks, but apparently, it does not go without saying. In September, two men walked onto the cone of Old Faithful at Yellowstone National Park. Not only is this dangerous, it can damage the geyser.
For their actions, they were sentenced to 10 days in jail, probation, a ban on visiting Yellowstone, and a fine. You can click this link to read more about what happened.