I found this story on Ludicrous Scenes's blog here. The original source was The Western Daily Press, July 1, 1871.
"A despatch from Dayton, Ohio, dated June 16, states that the Hon. Mr Vallandigham accidentally shot himself that day at Lebanon."
As the story goes, a man named Myers was shot in the abdomen and killed. A trial was taken up, accusing another man named McGehan of the murder of Myers. McGehan's lawyer was the Hon. Mr Vallandigham.
The Hon. Mr Vallandigham was trying to prove that it was possible for Myers to have shot himself–that there was reasonable doubt that his death was murder at all. During the trial, someone accused Mr Vallandigham of his theory being totally impossible. Myers could not have possibly shot himself.
"Mr Vallandigham took up a pistol from the table, saying he would show [everyone] in half a second. There were two pistols on the table, one of which was loaded, and he, by mistake, took up the loaded one, put it in his pocket, and withdrew it, keeping the muzzle next his body; and just as he was withdrawing it, the pistol went off, and shot the unfortunate gentleman in exactly the same part of the body where Myers was shot."
I have no idea if McGehan was acquitted or convinced, but I'm sure Mr Vallandigham's death in the middle of a courtroom was a powerful motivator for acquittal.