Here are different legal opinions selected from the internet 
"Both supporters and opponents of the death penalty should be concerned about the flaws in the system by which we impose sentences of death. More than 3,600 inmates sit on state and federal death rows around the country, while it becomes increasingly clear that innocent people are being put to death.
A 1987 study found that between 1900 and 1985, 350 people convicted of capital crimes in the United States were innocent of the crimes charged. Some escaped execution by minutes. Regrettably, according to researchers Radelet and Bedau, 23 actually had their lives taken from them in error."

Senator Feingold Seeks Moratorium Against Government Killings April 26, 2002 Posted by Staff under Progress Report, The Progress Report
 "...No system of justice can produce results which are 100% certain all the time. Mistakes will be made in any system which relies upon human testimony for proof. We should be vigilant to uncover and avoid such mistakes. Our system of justice rightfully demands a higher standard for death penalty cases. However, the risk of making a mistake with the extraordinary due process applied in death penalty cases is very small, and there is no credible evidence to show that any innocent persons have been executed at least since the death penalty was reactivated in 1976."  

Steven D. Stewart, JD, Prosecuting Attorney for Clark County Indiana, in a message on the Clark County Prosecutor website accessed on Aug. 6, 2008
The number of new death sentences has declined over the last several years in large part because people who serve on juries are increasingly choosing life without parole as an alternative to the death penalty because members of juries have read about so many mistakes in the system when innocent people have been convicted only to be exonerated years later.

Texas Moratorium network
As a consequence of the sensitivity of the criminal justice system to the due-process rights of defendants sentenced to death, almost two-thirds of all death sentences are overturned....
Like other human institutions, courts and juries are not perfect. One cannot have a system of criminal punishment without accepting the possibility that someone will be punished mistakenly. That is a truism, not a revelation. But with regard to the punishment of death in the current American system, that possibility has been reduced to an insignificant minimum.

Antonin Scalia, JD

DNA exonerations have created a learning moment, an opportunity to deal with the causes of wrongful conviction that victimize the innocent and allow the real criminals to go free: Mistaken eyewitness identification, false confessions, incompetent defense lawyers, poor forensic science and law enforcement misconduct. These issues can and must be addressed to prevent execution of the innocent. This is the heart of the death penalty moratorium debate. Reasonable people can differ about the morality of capital punishment. But it is not reasonable to excuse inequities in the administration of capital punishment. As the president has acknowledged, capital lawyers are not adequately trained or properly funded. Until the American Bar Association's Guidelines for the Administration of Capital Punishment are implemented, no citizen can be confident about the guilt of all death row inmates.

Barry Scheck
Co-founder/Co-director, The Innocence Project, www.innocenceproject.org. Written for The CQ Researcher, September 2005
Only a handful of capital cases involve genuine questions of innocence. By all means, we should put those few on hold as long as it takes to resolve the questions, and the governor should commute the sentence if a genuine doubt remains.

Kent Scheidegger
Legal Director, Criminal Justice Legal Foundation, www.cjlf.org. Written for The CQ Researcher, September 2005