Using iTunes as my main – and most likely only – media player started off quite badly. Not only that I had to change every single video I have into a *.mov-file, I also found *.mp3-files that would neither play on my iPod nor within iTunes. Trying to add these files via iTune’s automatic adding function would move them to the not added folder instead.
The MP3 Validator for Mac OS X neatly solved this issue for me. Just download the software and open it, before you can simply drag-and-drop your mp3-files into the program. It will show which failure persists and repair it. Just checked my whole library and found quite a lot of failures on files which I didn’t suspect in the first place.
PS: Windows-user might be happy to read that MP3 Validator is doing the same for them. 
PS:PS: Thanks to Trevin Chow for this helpful piece of information.
