We've all accidentally sat at a green light, right? You don't realize it's changed, and it takes someone honking at you to make you realize. I usually feel like a total jackass when it happens to me. I give the universal "sorry" wave and hit the gas, and that's it. End scene.
A guy recently honked at a cop in St. Louis who wasn't going at a green light, and this is what happened.
According to news reports (linked below) it's an officer in an unmarked car, not in uniform, and he's calling the driver who honked a "f-cking jackoff" and threatening to "tow your car and lock you up", among other things. And, he's got a pretty lengthy history of violence and bad behavior.
There's an investigation happening now, and hopefully some anger management classes in the future.