mp4 that can be played using Quicktime on PC but not on Mac <p>I have couples of .mp4 files, all of them can be opened and played using QuickTime Player 7.7 installed on PC.</p> <p>But some of them cannot be opened using QuickTime Player 10.1 on Lion.</p> <p>It said "The document xxx.mp4 could not be opened. This media may be damaged." And these unplayable files cannot be played on iPod too.</p> <p>All files are converted in same way in same format. </p> <p>What's wrong? </p>