In his IAC 2016 talk, Elon Musk said (at 28m 4s in the video) that the ITS booster tanks would use autogenous pressurization. This means there is gaseous oxygen resp. methane in the tank. To stay gaseous, its temperature must be above the boiling point.
On the other hand, the liquids are sub-cooled, i.e. their temperatures are far below the boiling point.
So we have "warm" gas and "cold" liquid in the same tank. How does this work out? Wouldn't the gas quickly condense, causing the pressure to drop? And respectively, wouldn't the liquid heat up and eventually boil?
I could imagine that it works for the booster, as the tanks are emptied so quickly that there won't be time to reach equilibrium anyway. But what about the spaceship? Is it also using autogenous pressurization? The diagram shows smaller tanks embedded in the spaceship tanks, could this be a helium pressurization system like in the Falcon 9?
Note: This web-based tool by NIST may be useful to determine and visualize thermophysical properties of fluids used in rockets.