The above song from Mass Effect 1 only plays twice and only for a few seconds:
1. When Ashley sees a man get impaled on a Husk spike.
2. In the five second cutscene on Feros that shows a Geth Destroyer walking down a hall.
What rare and/or unused music in a video game is your favorite?
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I can't remember what it was called, or what game it was from (Portal 2?) but there is this one fantastic piece that is only ever played once. [i]Once![/i] And after that you're likely to never hear it again in the game.