We show that phantom dark energy , if it is described by a K-essence theory , has three fundamental problems : first , its hamiltonian is unbounded from below .
Second , classical stability precludes the equation of state from crossing the “ Lambda-barrier ” , w _ { \Lambda } = -1 .
Finally , both the equation of state and the sound speed are unbounded — the first , from below , the second , from above — if the kinetic term is not bounded by dynamics .