One unlucky person lost 122Gb of music when Apple Music decided to 'tidy up' their music collection. If you use Apple Music, iTunes scans your MP3 and WAV files and looks for matches in Apple’s database. When there is a match, the file is deleted from your computer as Apple will now play it for you.
This has a problem; the matching process isn't fool proof. In this incident, one of the files that got deleted was a rare early version of Fountains of Wayne’s 'I’ll Do The Driving'. It was 'matched' to the later more widely available version of the song. A backup of all the music files existed, so it was possible to get them back.
The moral of this story is clear - make sure you have backups of everything in case software does more than you think it would. And make sure you check your backups. Many people don't and it is only when they come to use it that they find some files are missing or are corrupt.