Corruption among law enforcement officers is more prevelant than the replies are making it seem if you count not only criminal acts and of outright moral turpetude, but also graft, inequity, and various other subtle forms.
The most common form of what I would consider corruption is "fudging" facts on reports, in court room testimony, on citations, etc. The second most common form, I would say, is the system which often promotes a "code of silence", if you will. Many otherwise good officers will not report something a little bit grey or questionable, the situation persists, can grow, and encourages and "you cover my back-I'll cover yours" "good ol' boy" mentality.
There is considerable but very subtle inequity in the initial hiring process. How many law enforcement officers are cops because daddy or someone else in their family is a local business owner, mayor, city counsil-member, in certain civic or fraternal clubs, etc? I know several.
While "corruption" as in "on the take" or dealing drugs, etc is probably minimal (perhaps less than 3-5%) all this other subtle stuff I mention is rampant. I personally know a cop that got fired from one department for theft (of a fellow officers belongings in the locker room). His excuse was that it was a joke. He immediately applied and got hired on at another nearby local police department, his family high-up mucky mucks in the same church the police chief was a member of.
Another gal I know could not make it through one of the obstacles at the academy and never passed a night pistol qual. The obstacle at the academy was removed and her night pistol quals (I personally witnessed) were always fudged. She had a weirs pious attitude. Everyone hated her so much, the chief sent her for over two years to be attached to another joint task force to get her out of everyones hair because of all the complaints she generated. She never made a significant bust while on that assignment like her peers. Upon her return, she put in for sergeant and got turned down because there were three others with better backgrounds. She threatened to sue and was promoted. It turns out her dad is a former cop and ran his own little business and was in a certain fraternal club I won't mention the name of. I could tell you stories like this all day long.
One guy I went to the academy with is in prison for a sex crime I won't detail. One cop I worked with was later fired and went to jail for exposing himself to a minor. Another cop I worked with is (according to a tested reliable CI) running drugs in the area (meth) with his cousin. Another local cop was a known gang member when he applied. I could tell you stories like this all day. Anyone out there claiming to have been a cop for any length of time anywhere in the USA and saying they never saw any type of corruption is flat out lying!
Being former military, I have friends that are now cops coast to coast (Sacramento, Ca., Arlington, Va., Manhatten, Ks., Orlando, Fl.,) and I've gone on ride-alongs with them all and I've met cops from all over. I'm here to tell you now, in my training and experience (by the way, I graduated from my academy class with honors) close to half of the cops I've met should not be cops for one reason or another. Don't get me started on the judges!
I'm ashamed to have ever been a cop. I'm glad I can call myself an ex-cop and give citizens good advice on the matter. Be skeptical and suspicious. Keep at least a small audio recorder on you at all times. And be afraid. Be very afraid.
The Real War on Crime by Steven R. Donziger
Crime and the American Dream by Steven F. Messner and Richard Rosenfeld
Crime and Everyday Life by Marcus Felson
* There's no way you can go through a college level Deviant Behavior or Criminology course and not learn more than enough to sicken you (indisputable sick, sad, sorry, sadistic, social fact) about police corruption.