I found this story (in part) on Futility Closet's blog here. The original source was the Annual Register, 1822.
The Duke of Berry, the youngest son of Charles X of France, was assassinated at the Paris Opera in 1820 by an anti-royal bonapartist named Louis Pierre Louvel.
"A man of the name of Desjardins was tried on his own confession, for having admitted that he was an accomplice of Louvel, the assassin of the Duke de Berri.
"The case was clearly proved. Desjardins set up, as his defence, that he was so notorious for his falsehood, that nobody could give credit to a word he said, and produced a whole host of witnesses, his friends and relatives, who all swore to the fact with such effect, that he was declared Not Guilty."