In this talk I will describe some surprising features which arose from investigating strongly coupled QFTs with a first order phase transition using holography. Firstly, the existence of a simple formula for the velocity of domain walls between different phases with different pressures. The formula involves only perfect fluid hydrodynamics and effectively just the equation of state. We cross-check the result with holographic numerical relativity simulations. Secondly, we observe that domain walls in some holographic theories can be easily described on the boundary by an extended hydrodynamics. Thirdly, we use the two preceding ingredients to observe and analyze the appearance of hot plasma remnants, which do not cool down or nucleate bubbles in a boost-invariant expanding system with a confinement/deconfinement phase transition.