If you're trying to export a subtitled movie and see a message "The document "<your filename>.mov" could not be opened." after you've exported the movie, or if you double-click on the exported subtitled movie and get this message, the culprit is likely QuickTime X which cannot play subtitled movies. QuickTime X is the default QuickTime player installed with OS X Lion (10.7) and later.
You will need to install QuickTime 7 to play the subtitled movie*. More information about how to install QuickTime can be found here.
*NOTE: Due to security risks, we do not recommend QuickTime for Windows users (more on this subject here).