Michael Jackson's Death House Auction -- Secret Messages on Furniture

Several items from the upcoming Michael Jackson death house auction -- including a mirror from MJ's inner sanctum -- contain messages the public was never meant to see ... and now, TMZ has obtained pictures of the secret inscriptions.

The mirror reads "Train, perfection March April Full Out" ... and according to a rep for Julien's Auctions -- the company behind the sale -- Jackson wrote the motivational message to himself in the months leading up to his "This Is It" tour.

The chalkboard on the right contains a heartbreaking message from one of Michael's children -- "I love Daddy. Smile, it's for free!" Both the mirror and the chalkboard have remained undisturbed since the singer's death.

Clearly, neither message was intended for public viewing -- but we're told the items are hitting the auction block on December 17 ... as is.