well since the death penalty was brought back into play in 1977, there have been 100 exonerations based on DNA by the innocence project.
http://www.aclu.org/DeathPenalty/Dea...?ID=10119&c=65
As far as faulty evidence is concerned. Witness testimony is one of the most volatile types of evidence there are and yet the most convincing for a jury besides DNA. And sometimes you have a co-conspirator who will testify against their cohort in crime in order to get leniency and they may very well be the only witness to the crime. Of course you can see a conflict of interest there for determining guilt or innocence and there are alot of restrictions on admitting character evidence in court. You can't really bring crimes in unless they were perjury to prove a persons lying. I'm not saying that the system doesn't convict more innocents than guilty. I do still truly believe in our judicial system but there are faults, and when the penalty is death, I don't think "mistakes" are acceptable.