The last thing you want to see during an iTunes download is an error message. Fortunately, Apple is responsible about making sure to not double-charge users for a purchase. If you encounter a corrupt file download, there is a way to initiate a new download without needing to repay for the song.
A corrupted file is a song file that contains an error and cannot open or function in the way it was intended to function. The cause of file corruption is complex, but files are usually corrupted when something goes wrong during a save or download process. Corrupted files can rarely be repaired and should be erased and replaced with a good file.
Most iTunes files can be counted on to download easily and play accurately. From time to time, however, a corrupted version of a song file might be generated when you make an iTunes purchase. If that happens, go to the "Downloads" subfolder of the "iTunes Music" folder on your hard drive. There will be a list of "TMP" files, which are temporary files that only exist until the download completes. Delete the TMP file for the corrupted file. When you click "Check for Available Downloads" from the "File" tab at the top of iTunes, iTunes will download a new copy of the file at no extra charge.
iTunes will also allow you to import songs downloaded on third-party sites, such as peer shareware sites. If you encounter a corrupted file from a site other than iTunes, iTunes will not replace the file for free. You can either purchase a good copy of the song from the iTunes Store or download another copy from the third-party site.
If you are attempting to download a song on your iPhone or iPod Touch iTunes application, you won't be able to access or clear the temporary files. Try rebooting your device by powering it down and back on again, then downloading the file again. Alternatively, you can go to iTunes on your computer and go to the "Check for Available Downloads" link. iTunes will detect your mobile purchase -- as long as you have iOS 5 or later -- and download it for you.