In January of 897 Pope Formosus was put on trial and found guilty by the Cadaver Synod. More accurately, it was his corpse that was put on trial; the Pope died the previous year.

A painting of the trial, by Jean Paul Lauren, 1870

Pope Formosus (1816-1896) had a rather dramatic Papacy even before his posthumous trial and conviction. In 869 he was made Cardinal Bishop and, in 869 was sent to France by Pope Adrian II and Pope John VIII. But by 876 Pope John VIII lost trust in Formosus, and he was excommunicated. Two years later, in 878, Formosus was pardoned and reinstated. One of the conditions of his pardon was that he’d stay in exile, but even that was let go and he was fully absolved by Pope Marinus I, who restored him to his Cardnial Bishopric in 883. Then Formosus entered politics and encouraged King Arnulf, of the Franks, to invade Italy in order to liberate Rome from the Spoletan co-emperors Guy and Lambert, a father and son duo of emperors. Unfortunately, Arnulf suddenly became paralyzed and had to retreat to Germany. Soon after Formosus himself died.

Pope Formosus, drawing, 1588

After his death Formosus’ enemies, under the leadership of Pope Stephen VI, had his corpse exhumed, dressed in his vestments, and put on trial. A Deacon answered for the corpse, but failed to provide adequate defense. Pope Formosus’ election was declared invalid and all of his actions were overturned. Further, his fingers of consecration were cut off. His corpse was buried, then dug up and thrown into the Tiber, only to be found by a monk when it washed to shore
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But the story doesn’t end there: Pope Theodore II reinstated the guilty Cardinal yet again, and reburied the body in 897. Finally, in 904, Pope Sergius III once again overturned Pope Theodore II’s verdict and ordered that all who had been ordained by Formosus had to lose their ordination. This is where things stand now, but you never know, the story may have another twist or turn in it yet.

Further reading:
http://www.newadvent.org/cathen/06139b.htm

http://www.rapiddiffusion.com/history/pope-formosus-dead-mans-trial/