: Another 2010 leak, "Forever" showcased a slightly more mature vocal style as Bieber's voice began to change during puberty.
During the production of My World 2.0 , several high-profile artists and writers recorded "reference tracks" for Bieber to follow. Many of these have since leaked, providing a "behind-the-scenes" look at the album's creation:
: Collectors often upload unreleased playlists, such as those on Bieber Industries Unreleased Music .