This is from history.com, This Day in History February 23, 1885
A remarkable reprieve for a man sent to the gallows
On this day in 1885, a 19-year-old man named John Lee is sent to the gallows in Exeter, England, for the murder of Ellen Keyse, a rich older woman for whom he had worked. Although he insisted he was innocent, Lee had been convicted and sentenced to death by hanging. However, after the noose was put around his neck and the lever that would release the floor beneath his feet was pulled, something malfunctioned and Lee was not dropped. Strangely, the equipment had been tested and found to be in working order. In facts, weights used in a test run plunged to the ground as expected. The hanging was attempted two more times, but when Lee stood on the trap door, and the lever was pulled, nothing happened. He was then sent back to prison.On November 15, 1884, Keyse, who had been a maid to Queen Victoria, was found dead in a pantry next to Lee's room. Her head was severely battered and her throat cut. There was no direct evidence of Lee's guilt; the case was made solely on circumstantial evidence. The alleged motive was Lee's resentment at Keyse's mean treatment.
The authorities, mystified at the gallows' inexplicable malfunction, decided to ascribe it to an act of God. Lee was removed from death row, his sentence commuted, and he spent the next 22 years in prison. After he was released, he emigrated to America. The cause of Lee's remarkable reprieve was never discovered.
Condemned prisoners no longer have a chance at such reprieves. Even when there are mishaps in carrying out an execution (in one case, an executioner failed to properly find a vein for a lethal injection), authorities follow through until the prisoner has been put to death.