Go to the part where Obi Wan is fighting General Grievous's guards. Notice where he makes the stone fall. A short while after, while General Grievous makes his initial attack, look where the stone was dropped. It's not there anymore. It just disappeared.

Later on when they do a close-up with grevious & obi-wan's eyes, obi-wan pushes grevious into it, back up on the ceiling!!!