Williams is one of the most notorious defensive coaches ever because of one illegal scheme. He was the mastermind behind BountyGate, a concept that almost cost him his career. Sean Payton suffered a one-year suspension and Williams almost got a lifetime ban for this villainous practice (via NBC Sports).
In short, Williams offered his players bonuses if they targeted and injured opponents. This rocked the NFL because it was so immoral. However, it emerged that this was nothing new because Williams did the same thing in his previous jobs. Amazingly, he continued to bounce around the NFL and even had an interim head coach job.