A review of death penalty convictions by the federal government has turned up 27 instances in which "exaggerated scientific testimony" from FBI forensics experts may have played a role. It's unclear how many times such testimony led to false convictions or could nullify correct convictions the Washington Post reports. However the assessment did lead to a last minute stay of execution in May.

The testimony is the "once-widespread practice" where experts said that hair found at crime scenes could be used to identify suspects:

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Since at least the 1970s, written FBI Laboratory reports typically stated that a hair association could not be used as positive identification. However, on the witness stand, several agents for years went beyond the science and testified that their hair analysis was a near-certain match.

While it has long been speculated that proof of an innocent prisoner's execution would torpedo the death penalty in this country, the evidence doesn't necessarily back that up. For example, this article about the potentially incorrect execution of Cameron Todd in Texas has not stopped the state from continuing to execute prisoners.