Sometimes when playing a QuickTime movie -- .mpeg or .mp4 file -- you get an error stating that you have a "Bad Atom" or "Bad public movie atom." Atom files are a component of movie files, which when all are added together, become the one .mpeg or .mp4 file that you view in QuickTime. When you copy a movie file from one location to another, the internal files can be corrupted or moved out of sequence, causing this. QuickTime is a precise application that takes slight errors very seriously and will not play a file it thinks is damaged.
Download the VLN media player application from VideoLAN and either RAR for Mac from Softonic's RAR or Stuffit (see Resources). Install the compression tool you chose and VLC using the installer files you just downloaded.
Compress the movie file that you are getting the Atom error from using the compression tool you just downloaded. Simply right click on the movie file and you should have a "Compress" or "Zip" option in the menu that pops up. Then uncompress this compressed file. This forces the files in the compressed archive to be reordered when written back to your hard drive and can resolve the Atom error.
Open the QuickTime application and try to play the movie file again. If you still continue to get the error, you have two options. You can either try compressing and uncompressing the file again (it can take two or three tries sometimes) or you can try to play the movie using the VLC application, as VLC has a much higher tolerance for errors than QuickTime does.
Open the VLC application if the compressing option did not work for you and browse to your movie file. Open the file using VLC and this should play the movie. If the movie still does not play, then there are corruptions in your movie file and you will need to obtain another copy, hopefully without getting any corrupted Atoms along the way.