From personal experience, I know that employees of the studios (where the master tapes are usually stored while there's a project in process) will, when bored during graveyard shift, slap on a master tape of any particular artist and make their own mixes of the songs. I know of one in particular who made three different mixes of the song that became "Talk To Me" about a year before Rock A Little was released. (of the cassettes I heard of them, the final "official" release sounded pale in comparison)
And, also at this same studio, there was a "safety tape" of the master tapes of the Beatles' "Sgt Pepper" album...the overnight tech crew made their own mix of the album...and my oh my how different it sounded!!
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