So... Here I was playing sets on my new Traktor S4 on my 2010 iMac, and noticed that Traktor was acting weird--comments were getting erased, ratings were disappearing, play counts weren't being kept. In other words,Traktor's explorer worked on some files and not others.
I tried changing file permissions, reinstalling Traktor, etc... nothing solved the problem... Doing research didn't show me any concrete solutions (e.g. change the play count settings; this didn't work).
Then it occurred to me, I haven't defragmented my Windows PC in over 2 months. And a few weeks before, I had my windows PC sync all my mp3 files to my iMac.
So long story short, the mp3 files were messed up ("garbage at the end of file")...