The event 'onmousemove' will fire on page load even if the mouse haven't actually moved. In the example provided in the URL field, the 'Mouseover box.' is bound to 'onmousemove', and whenever it's fired, 'Mouse moved.' box is shown. If you reload the page while your mouse is over the 'Mouseoverbox', the 'onmousemove' event will fire, even if you didn't move your mouse. This only happens in safari.
This isn't a JSC bug, switching components, hoping it'll find some experts in this area.
See also: bug 26401.