You're probably thinking of the story that goes viral every few months, Gary Plauché who killed his son's abuser in 1984 and with the help of pychiatrists reports showing he was having a psychotic break and wasn't fully mentally competent at the time, plead guilty in return for a 7 year suspended sentence, 5 years on probation and 3,000 hours community service.
He wasn't found not guilty.
Sometimes there's other fringe cases where a state prosecutor decides not to prosecute (normally for political reasons) but when people who obviously commit crimes go in front of a jury, they get found guilty, because that's a juror's job, and giving a false verdict is a crime.
No it was a different case but I was going to cite that one as well. There was no doubt he was guilty as he shot him as he was being transported by detectives at an airport.
I know that's the letter of the law and the criminal implications of doing so, but juries have, normally in America. Actually this conversation brings to mind the OJ Simpson case. At the time he was a national hero and celebrity in America from his football and acting careers and was found not guilty when it was pretty obvious he'd done the murders.