Believe it or not, the galley carts and the smaller silver carriers (the square boxes) are 'no-go' items. I have been told a few different reasons for why (weight and balance, that sort of thing), and am not sure what is actually correct...but technically the plane isn't 'legal' to fly without the full compliment of carts. I have been told that it must be all carts or no carts, or it is no go.