Apple patched QuickTime not once, but an amazing eleven times on Wednesday. Some of the patches were to improve stability and compatibility, but there were also some exploits which were found and patched.
“It's at least the sixth time Apple has patched QuickTime since October, as researchers and hackers have been closely examining media players for vulnerabilities. As operating systems have become more secure, vulnerabilities in applications have become a favored route to break into a PC.”
Apple is notoriously coy with information regarding the specific exploits, but the running rumor is that most of the exploits patched were actually found by Apple themselves.