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It's a cube... It's supposed to be flat 
I think the 1x1 black pixels you use to make the "bolts" could have been made better. You can't really see what they are supposed to be, especially because three of them are on/close to dark background, so you really only see one unexplained pixel in the top left corner. Maybe make them 2x2 or remove them completly?
Also when the ? dissapears on the left side, it should just continue to be white. It's on the sunside so white stuff just stay white.
ahh...the good days of the 1st mario ^ ^