What? The posthumous trial of Pope Formosus.
When? 897 AD.
Why? Formosus’ successor’s successor— Pope Stephen VII—accused the rotten corpse of perjury, among other fun things. Formosus’ remains were found guilty and, like the calm and rational person he was, Stephen took Formosus’ right hand—the one he used to give blessings—and chopped off three of his fingers. The whole ghastly spectacle turned public opinion against Steven, and shortly afterwards he was sent to prison. He was strangled to death soon after, because fuck that guy.