Windows 7 build 7000, which leaked less than two weeks ago, has a data corruption issue when playing back MP3s using Windows Media Player 12. The problem only happens under the following circumstances: the first few seconds of an MP3 file will be cut if the header of the mp3 file is larger than 16 kilobytes and if metadata is written to the file. The corruption will occur either when the user edits the metadata from inside WMP12 or Explorer, or if WMP12 is set to automatically fill in missing metadata using the online service.

The changes are irreversible, so if you plan on playing back your MP3s with WMP12, make sure to back up your music library. Microsoft is currently hard at work readying a Quick Fix Engineering (QFE) patch that will be distributed by Windows Update. A Microsoft employee has confirmed the bug and said that the company will be addressing the problem in the beta program. While many builds have leaked so far, most beta testers still haven't received builds to test.

Full Story: http://arstechnica.com/journals/microsoft.ars/2009/01/05/warning-wmp12-beta-could-corrupt-your-mp3s