Mary Surratt was sentenced to hang, but it is believed that the sentence was to flush out her son John, a Confederate courier, who had been part of the plot to kidnap Lincoln. The belief was that the son would not sit idly by and allow his mother to be hanged. Well, he did. When John Surratt was captured a few years later, he stood trial in a civilian court which resulted in a hung jury. Since the statute of limitations on conspiracy had expired, he was released. John died in 1916, and often made money giving lectures on the Lincoln assassination.
He escaped to Canada, made his way to Europe, lived abroad for a few years and was eventually recognized and extradited from Egypt. After two months of testimony, Surratt was released after a mistrial; eight jurors had voted not guilty, four voted guilty.
The other thing which aided him was that this was a criminal trial by the State of Maryland, not a military tribunal which condemned his mother and three co-conspirators to the gallows. A recent Supreme Court decision had declared the trial of civilians before military tribunals to be unconstitutional (Ex parte Milligan).