U.S. v. Boone - using evidence of prior misconduct in a use of force case
This was a case brought in federal court against a police officer accused of using excessive force during the arrest of an individual. There are several issues, but I want to focus on what is called "404(b) evidence." Typically a person's prior conduct cannot be used against them in trial to prove "they did it before, so they probably did it again." But Rule 404(b) DOES allow evidence of prior bad acts to be used for the purpose of showing "motive, opportunity, intent, preparation, plan, knowledge, identity, absence of mistake, or lack of accident."
In this case, the officer was alleged to have taken a running start and kicking a defendant in the face when he was pinned to the ground by other officers, yet still struggling. The kick caused the loss of two teeth, damage to another, a broken nose, and a laceration requiring stitches. During the trial the government was allowed to present proof that the same officer had injured another arrestee years earlier using unnecessary force. This was for the purpose of proving that the recent injury was not a mistake or an accident. The Court of Appeals allowed the testimony to be used, and ultimately the officer was sentenced to 63 months in prison.
The take-away is twofold: first, cops don't always "get away with it" (though bad cops are far fewer than most people think; it's like any other profession, there good ones and bad ones); and second, having a lawyer who knows the rules is important, so if you are charged with a crime, consult a defense attorney.