This actually happened to me once. I was climbing out of a girlfriend's bedroom window (she lived with her folks who didn't know) when a rookie cop trying to make it big jumps out of the bushes. He was scruffy, wearing plain clothes and had cotton gloves without fingers. He looked VERY sketchy! There were no police cars around and no other officers, nobody at all in fact. He did not have a radio. He approached me and told me he was arresting me for B&E.
He pulled his badge out and told me he was an officer. He told me to place my hands behind my back so he could handcuff me. Thoughts of being stuffed into the back of a windowless van and being but raped and tortured to death entered my mind. I told the guy "Fuck that" you are not a cop. He proceeded to beat me in the head with his magnum flashlight at which point I lost it and hit him with so many lefts he was begging for a right. I destroyed his face, two black eyes and a broken jaw! Quickly after subduing him, I ran back to the girl's house and pounded on the front door.
I called 911 and said there was someone impersonating an officer. About 20 minutes later the cop I beat up and his Sergeant were at the door. He told me "Sorry for my officer's misconduct." That asshole cop followed me around for weeks trying to bust me on some stupid traffic issues. I can only guess the shit he had to put up with in the office after this. I got away without any visible damage. Anyway let this be a lesson to you cops out there - don't think the badge is the be all and end all of power, and it definitely doesn't make you a tough guy.