We extend previous work showing that violation of the null energy condition implies instability in a broad class of models , including gauge theories with scalar and fermionic matter as well as any perfect fluid .
Simple examples are given to illustrate these results .
The role of causality in our results is discussed .
Finally , we extend the fluid results to more general systems in thermal equilibrium .
When applied to the dark energy , our results imply that w = p / \rho is unlikely to be less than -1 .