  We show that self-interacting vector field theories exhibit unphysical
behaviour even when they are not coupled to any external field. This means any
theory featuring such vectors is in danger of being unphysical, an alarming
prospect for many proposals in cosmology, gravity, high energy physics and
beyond. The problem arises when vector fields with healthy configurations
naturally reach a point where time evolution is mathematically ill-defined. We
develop tools to easily identify this issue, and provide a simple and unifying
framework to investigate it.
