My only bad cop story was when I was in college. My roommate had just had some oral surgery and was pretty drugged up for the rest of the day. He needed to go to Walgreens late at night to pick up some ice cream and some ice packs.
I took him and as we were leaving he apparently walked through the entrance way instead of the exit way and had to maneuver around the cop that was entering (this wasn't intentional, he was pretty loopy from the drugs and not the kind of guy that starts trouble). The cop shoved him from behind and yelled something about showing respect. My friend was on a good dose of vicodin and still woozy from whatever he had during surgery. He looked real confused and didn't say anything. The cop grabbed him by the neck and slammed him up against the brick wall. I thought maybe the cop thought he was drunk so I kind of jumped in and told him he's just out of surgery. The cop told me to shut the fuck up or I'd be arrested.
The blow against the wall blew open the stitches in his mouth and he started bleeding pretty good from there. I think the cop realized he had gone too far and let go and told us to get the hell out of there. We went to my car and I was trying to get something for him to catch the blood in. The cop came storming out there then and told us to go straight home or we'd be arrested for trespassing and resisting arrest. I told the guy he needs a doctor as his stitches look to be busted open. He told us we better go home and not to think about going anywhere else. The cop tailed us all the way back to our apartment.
The next day he had to get some stitches put back in. He also had a pretty big scrape along one side of his face from the brick wall. He filed a complaint but the cop denied everything and claimed he was drunk and fell down on the pavement. They didn't really take it very seriously anyway.
It's not a big deal either now that I'm in Chicago. That kind of stuff here is normal for cops, usually much worse. This cop beat the shit out of some innocent bartender while his cop buddy looked on. They didn't charge him (despite the tape) until it hit the media a couple months later.
YouTube - Chicago Cop Beats Bartender - abc news
They also have a pretty good history of beating the shit out of women, drunk driving and killing people (then covering it up despite video tape evidence), and torturing suspects.
YouTube - Chicago Police beat woman
Tough to give them the benefit of the doubt when so much mounting evidence keeps showing up with them abusing their powers.