A study is presented concerning an experiment under weightlessness. A pure fluid (SF6) is heated near and below its critical point, where liquid coexists with vapor. After the temperature rise, the vapor phase passes well beyond the temperature of the heating walls. This surprising finding is discussed in the light of an adiabatic heat transfer or "Piston effect" and the special geometry of the liquid and vapor. In addition, the shape of the gas-liquid interface is distorted near the solid wall by the thrust of vapor production ("recoil" force), a precursor to the well-known boiling crisis in heat exchanger.