The only reason I knew this was because I've been through it before. But not in relation to MP3s, in relation to creating audio CDs. I had helped a friend make a custom audio CD out of tracks he had supplied, and his software (SoundForge, as it happened) also gave the option to append text to the end of the WAV file, which he blissfully activated without knowing the consequences. We burned the CD (a rather involved process back in those days with an ancient first-generation CD recorder) only to discover the click at the end of each track.