I had to look it up but quoted from Columbia University:

Of the 5,760 death sentences imposed in the study period, 4,578 (79%) were finally reviewed on "direct appeal" by a state high court. Of those, 1,885 (41%; over two out of five) were thrown out because of "serious error," i.e., error that the reviewing court concludes has seriously undermined the reliability of the outcome or otherwise "harmed" the defendant


Of the death sentences that survived state direct and post-conviction review, 599 were finally reviewed in a first habeas corpus petition during the 23-year study period.37 Of those 599, 237 (40%; two out of five) were overturned due to serious error.38
and

The seriousness of these errors is also revealed by what happens on retrial, when the errors are cured. In our state post-conviction study, an astonishing 82% (247 out of 301) of the capital judgments that were reversed were replaced on retrial with a sentence less than death, or no sentence at all.44 In the latter regard, 7% (22/301) of the reversals for serious error resulted in a determination on retrial that the defendant was not guilty of the capital offense.
40% of cases reviewed brought forth errors that seriously altered the outcome of the sentence. Proof that not all individuals convicted of crimes and sentenced to death are deserving of the sentence. The cases we hear about are typically televised but there are many that fall under the radar and due to the lack of proper representation are placed on death row.