Models of dark energy are conveniently characterized by the equation-of-state parameter w (the pressure divided by the energy density). If w < -1, the energy density increases as the universe expands, the dominant energy condition is violated, and the vacuum can become unstable. Despite these worries, there is no theorem which conclusively excludes the possibility that w < -1. I will discuss the possibility that the dark energy is a scalar field with a negative kinetic energy; such a model might be phenomenologically viable, if only as an effective field theory valid up to a certain momentum cutoff. Under optimistic assumptions, the instability timescale can be greater than the age of the universe only if the cutoff is at or below 100 MeV. Therefore, observers should keep an open mind, but the burden is on theorists to demonstrate that any proposed new models are not ruled out by rapid vacuum decay.