If you're not expecting that, and have mostly empty or invalid ID3 information, because you have habitually kept metadata in the filename and directory hierarchy, iTunes will destroy your library. Everything becomes Unknown Title 243 by Unknown Artist 14, and by the time you notice (because who's expecting an MP3 player to rearrange all the files on disk!?), the only way to fix this is by listening to each track and trying to figure out via Google what song it is.

This happened to me in 2003 when I switched from Linux to Mac. I still get a little worked up over that, even though nowadays I just let iTunes manage everything and throw away the original files after I rip something.