You can get this error for a variety of reasons?the format isn't supported, you aren't authorized to play a file, etc. Usually if your syncing a popup box will appear in iTunes with a list (you may need to expand it by clicking a small + sign in the popup) of what the problem files are (not sure if that happens when using the new wifi sync). If you've dismissed that in the past, you may need to use the Reset Warnings option in iTunes. That should give you back a list of problem files.