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On the Beach in the northern part of the Western Continent, in a hole in the ruined structures in the area, angling the camera correctly will reveal an out-of-bounds picture of an Asian woman. This person was world builder Fred Selker's girlfriend at the time of the game's development, and its placement at the northern-most/highest area on the world map could be a gesture of affection towards her.
Contributed by MehDeletingLater
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The unlockable solo 'Bring It On Home' summons a fiery zeppelin that crashes into the ground when played. This is a direct reference to both the Led Zeppelin song of the same name and the burning zeppelin on the cover of their debut album.
Contributed by BananaFoam
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Originally the famed vocalist Ronnie James Dio was meant to be the voice of Lord Doviculus. However as the game went further in development the role was given to actor Tim Curry. Tim Schafer explained that "Anyone who has seen his amazing performance as the Lord of Darkness in the movie Legend knows why we cast him in the role of Doviculus, Emperor of the Tainted Coil."
Contributed by Hipserious
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The saving icon in the game changes on certain days to remember several fallen pioneers in heavy metal music.
On February 19th the saving symbol changes to a lighting bolt. A reference to former AC/DC vocalist Bon Scott who died on said date.
The same also applies to Randy Rhoads (former guitarist for Ozzy Osbourne) and "Dimebag" Darrel (guitarist for both Pantera and Damageplan) where on March 19th and December 8th, both dates on which the respective musician died, the saving symbol changes to a "PR" and a dime respectively.
Contributed by Hipserious