It happens semi-often on my proof-of-concept embedded in a .mov
<http://remahl.se/david/vuln/018/demo.html> in Safari on my
PowerBook G4 Titanium (1GHz). Some other people have said that they
have the same problem. Reloading the page once or twice fixes the
issue.
Look in the console log to see if you have any messages from the
Quartz Composer QT component