While working on a bit of video that I needed to minimize and view in the Dock, I discovered that it was suddenly impossible to do so. Rather than the smooth video I’m accustomed to seeing in the Dock, I saw a single still frame yet could still hear the audio track. When I attempted to expand the video to full size by clicking on its docked counterpart, nothing happened.
A search of the Apple Discussion Forums revealed the answer. This behavior will result if you install DivX 5. Specifically, the DivX 5.component stored in /Library/QuickTime mucks things up. To make QuickTime behave as it should, remove this component. Before doing so, note that once you do this those movies that depend on DivX will no longer play back properly.