Execution, the ultimate punishment, but what if it was a mistake?

It seems there are now serious doubts, even amongst the victim's family, that the man executed for murder may not actually have committed the crime. There have been a number of cases in the past where alleged murderers held on 'death row' have been exonerated prior to the punishment being carried out, but according to this New York Times article this is the first occasion that major doubts have arisen post-execution. The case occurred in St Louis. I'll be trying to follow the progress of this investigation.

I have always been opposed to the death penalty, simply because it is impossible to rectify errors. Another factor is that I believe firmly that if murder is wrong, then judicial murder (for that is what executions represent) is wrong, too. On the other hand I think custodial and other punishments can often be too lenient and/or not geared effectively to rehabilitation, for those whom this is thought to be a feasible option, even if there are some convicted criminals who should never be released (in my opinion).