Manifestly covariant canonical quantization III: Gravity, locality, and   diffeomorphism anomalies in four dimensions
  The recently introduced manifestly covariant canonical quantization scheme is applied to gravity. New diffeomorphism anomalies generating a multi-dimensional generalization of the Virasoro algebra arise. This does not contradict theorems about the non-existence of gravitational anomalies in four dimensions, because the relevant cocycles depend on the observer's spacetime trajectory, which is ignored in conventional field theory. Rather than being inconsistent, these anomalies are necessary to obtain a {\em local} theory of quantum gravity. 
T. A. Larsson