The Norwegian mass murderer dressed up as a police officer. I’ve read about other criminals doing this. Augmented vision technology could make it impossible to fool someone else into thinking that you are a policeman.
Imagine a future in which we all have glasses ( or VR inducing nanobots) that provide us with additional information about our environment. It would be trivial for all uniformed policemen to have digital badges that sent out time encrypted signals which could prove to viewers that they were real policeman.