EDIT: Why the downvote? Can't handle the truth? Here's some evidence that, at least in DC, the law is used to effectively arrest people for resiting arrest, with no other charge:
"Nearly two-thirds of those arrested [in the District of Columbia] for assaulting an officer weren’t charged with any other crime, raising questions about whether police had legal justification to stop the person."[1]
"In Coghill v. United States, 982 A.2d 802 (D.C. 2009), the court upheld a conviction for misdemeanor APO. Mr. Coghill was stopped by police while driving a car, and refused to let police search his car. He then got out of the car at their instruction, but at some point got back in the car. Officers tried to drag him out of the car, but he braced himself against the floorboards and gripped the steering wheel."[2]
"On Tuesday, Ward 3 DC Councilmember Mary Cheh introduced a bill to reform some elements of criminal justice procedure. It would change the law around "assaulting a police officer," strengthen prosecutors' duty to turn over evidence to defendants" [3]
[1] http://wamu.org/projects/assault-on-justice/ [2] http://greatergreaterwashington.org/post/10619/assaulting-a-... [3] http://greatergreaterwashington.org/post/26688/mary-cheh-wan...
Is it? How hard would it be for him to "find" enough drugs in your car to charge you with felony possession? Or charge you with assault against a police officer? (what assault? The one that he and his colleagues just "witnessed" while you claim you were handcuffed to a chair - who is the jury going to believe? You or 5 uniformed police officers who "saw" the whole thing?) Better hope you don't "steal" his gun and end up dead in a self-defense shooting.
There's a lot that a local cop can do to make your life miserable if he's not bound by law (or ethics)
Don't count on police cameras to keep you safe since they can "malfunction" when the video they capture may be inconvenient like "officials say all seven dashboard cameras in the police cruisers coincidentally malfunctioned." (https://www.washingtonpost.com/news/the-watch/wp/2014/03/26/...)
This is one place where ubiquitous surveillance can be a good thing - police cameras (body cams, dash cams, and police station cams) should be unalterable (with footage signed by a neutral 3rd party with civilian oversight) and non-disablable, if a police camera is turned off or is blocked, it should send out an "officer assistance" signal by radio, and courts should view police actions skeptically when they happen outside of camera view.
Release of footage can still be governed by the courts to protect suspect privacy, but there's no reason that a police officer's official actions should be kept from the public.
[0] http://www.theguardian.com/us-news/2015/feb/24/chicago-polic...
Witnesses for the FBI sometimes find themselves in the predicament of having to either agree to a heavily "edited" version of their testimony - or face felony charges.
I would agree that it's not a bad idea to have an attorney present in either case - they have the effect of "keeping them honest", but you can decide to "help" a local detective with very little risk of you being made into a felon. Not so for the FBI.