When the state of Florida executed Bennie Demps by lethal injection a year ago, it did not go smoothly.
Execution officials had a problem finding a vein
"He complained about the procedure and said that he bled profusely," said George Schaefer, attorney for Bennie Demps.
What witnesses saw was Demps strapped to the gurney and screaming, "They butchered me" before he finally succumbed.
In a separate incident in Texas, execution, defense attorney Karen Zellars vividly recalls the lethal injection of her client Stephen McCoy 12 years ago.
"He lurched forward, pulling the gurney up with him," she said. "It was so violent that a news reporter behind me passed out cold on the floor."
Despite several documented problems, executions done by lethal injection is considered more humane than the hangman's rope, the gas chamber or the electric chair.
"It is less subject to mistake," said Rory Little, Hastings College of Law at the University of San Francisco. "It is less subject to glitches."
The Lethal Chemical Cocktail
Three chemicals are commonly used: Sodium pentathol to induce deep sleep, pancuronium bromide to paralyze the muscles, and potassium chloride to stop the heart.
Of the more than 500 lethal injections done in the U.S. since 1982, death penalty opponents can point to only about 20 cases that have gone wrong. But when it happens, they say, it can be torture.
The Hippocratic oath bars doctors from administering the fatal chemicals. So the task falls in the hands of prison employees— resulting in instances of incorrect dosages that have allowed inmates to awaken during their execution.
"It would look as though he were completely comfortable and lying still," said death penalty opponent Dr. Edward Brunner from Northwestern University Medical School. "The sleep drug could wear off and he could be completely aware of what's going on … He could be awake, but paralyzed and suffocating."
Feds Not Worried About McVeigh Execution
Yet federal officials are not worried about mistakes on Monday.
"We believe the procedures that we have set up will be effective and not cause undue pain to the inmate," said Greg Hershberger from the Federal Bureau of Prisons.
But should the public even care about the undue pain for someone such as Timothy McVeigh?
When serial killer John Wayne Gacy was executed in 1994, he needed two rounds of injections.
"He got a much easier death than any of his victims," said William Kunkle, former Cook County, Ill. prosecutor. "In my opinion, he got an easier death than he deserved."
Lethal injection may well be the most humane, but as one death penalty advocate put it, there is no easy way to put someone death. ABCNEWS' Dean Reynolds contributed to this report.