Owners of iPod shuffles have long had the option of converting songs to a lower bit-rate--128 Kbps--when they sync with iTunes. Lower bit-rates makes for smaller files, which means more songs on the shuffle. As of iTunes 9.1, that same tool became available to owners of other iPod models, as well as iPhones.
But MacOSXHints.com founder and editor-emeritus robg ran into a problem when he tried to enable that on-the-fly downsizing for his space-challenged 16GB iPhone: "Every single time I synced the iPhone, iTunes would re-copy every single song--basically redoing the conversion, which is obviously a time-consuming process."
Thanks to some of his helpful Twitter followers, Rob found the cause--and the fix. It turns out there are two versions of iTunes 9.1.1: 9.1.1 (11) and 9.1.1 (12). (Open iTunes -> About iTunes to find out which one you have.) If you have (11), you want to download (12) from Apple's iTunes page, because it fixes the re-syncing problem.
More specifically, 9.1.1 (12) fixes the cause of the problem: In 9.1.1 (11), iTunes changes the Modification Date of a song every time you play it; when syncing your iPod or iPhone, it therefore thinks those songs need to be reconverted to 128 Kbps. 9.1.1 (12) no longer changes the Modification Date field after every play and so doesn't reconvert those files every time.
This story, "Prevent Excessive Resyncing in ITunes" was originally published by Macworld.