In short, the federal courts will not interfere in an execution on the basis of new evidence of innocence. In Texas, any new evidence must be presented to an appeals court within 30 days of conviction, or it will not be considered admissible. But the evidence came long after the 30-day limit, and Herrera was executed. O'Dell was executed in 1997. This law severely restricts the writ of habeas corpus, both in the number of appeals that can be made and by requiring that any federal habeas petition must be filed within six months of state court proceedings.
Innocence is merely a red herring.
The fact that people are wrongly convicted and sentenced to death is indisputable. And we know that mistakes are typically uncovered after many years of combating the system, frequently after many appeals have failed. The work is often by defense lawyers working pro bono or journalists or other concerned citizens with no formal role within the system. The notion that these haphazard mechanisms have exonerated and will continue to exonerate all of the wrongly convicted is naive. The second type of response is that it is unreasonable to halt executions because of the possibility of mistakes.