But how large is the risk of a false positive match to an existing suspect? To an innocent individual culled from a database? By and large, we are limited to isolated reports in newspapers--reports that are newsworthy precisely because they are rare. The most complete compilation of the troubling cases, presented in a survey of the ways that errors can arise, is to be found in a book chapter by Bill Thompson of the University of California at Irvine. 
Professor Thompson is an unusually knowledgeable and astute commentator, consultant, and advocate in the field of DNA evidence, and it should be revealing to work through his examples. That is what I have started to do. So far, I have looked into only the very first case noted in the chapter. According to Professor Thompson, it exemplifies a "common problem" [1, p. 230] of "[a]ccidental transfer of cellular material or DNA from one sample to another" [1, p. 229] causing "false reports of a DNA match between samples that originated from different people" [1, p. 230].
The example is a 1988 DNA test in a rape case in Scotland that led to the conviction of Brian Kelly. Thompson simply reports that "Scotland's High Court of Justiciary quashed a conviction in one case in which the convicted man (with the help of sympathetic volunteer scientists) presented persuasive evidence that the DNA match that incriminated him arose from a laboratory accident" [1, 230]. The "accident" in question consisted of DNA leaking from one well to an adjacent one in an agarose gel used in VNTR typing or an analyst's misloading some of the same DNA sample into both wells instead of just the one she was aiming for.
But the evidence that Professor Thompson found "persuasive" did not persuade the court. Indeed, the experts did not even testify that leakage or misloading had occurred. Rather, they stated that it was a "low risk" event, that the possibility could not be excluded, and that a procedure that would have reduced the risk could have been followed (and was adopted two years later) [2, ¶¶ 15-17].
Thus, the Scottish Appeals Court, noting other evidence in Kelly's favor and weaknesses in the Crown's case, quashed the conviction--but not because it concluded that that the match was false. The court quashed the conviction because the jury was not informed of the fact that the same DNA could end up in two adjacent lanes. The court wrote:
It was not suggested that there is evidence positively indicating that cross-contamination did, or may have, occurred. On the basis of the evidence tendered by the appellant, it is maintained, on the other hand, that there was a risk of cross-contamination arising from the practice at that time of using adjoining wells for DNA samples from the crime scene and the suspect, and of such cross-contamination being undetected. It was not in controversy that it was possible for there to be leakage between adjoining wells or for DNA material to fall accidentally into a well next to the one for which it was intended. Up to a point the evidence ... as to the procedures which were followed, and the special care which was taken, countered the risk that such a mishap would in practice occur or be undetected. However, such evidence does not in our view provide a complete answer. In particular there was, on the evidence, a risk that the leakage of DNA from the well for the suspect's reference sample to the adjoining well which already held the crime scene sample would not be detected. It was, of course, a low risk, but it was of sufficient importance to be recognised by experts ... .
... In our opinion there is evidence which is capable of being regarded as credible and reliable as to the existence of a risk of cross-contamination occurring without it being detected. The risk was a low risk. It may be that in other circumstances the fact that the jury did not hear such evidence would not lead to the conclusion that there had been a miscarriage of justice. However, in the present case it is otherwise since the DNA evidence was plainly of critical importance for the conviction of the appellant. If the jury had rejected that evidence there would, in our view, have been insufficient evidence to convict the appellant. Accordingly, while the evidence related to a low risk of cross-contamination, the magnitude of the implications for the case against the appellant were substantial. For these reasons we have come to the conclusion that the appellant has established the existence of evidence which is of such significance that the fact that it was not heard by the jury constituted a miscarriage of justice. [2, ¶ 21-22]
Based on this opinion, the 1988 DNA testing with a superseded technology is a far cry from is a true example of an innocent man convicted because of "a laboratory accident." It is nothing more--or less--than a case in which the defendant did not present expert testimony at trial that the laboratory used a procedure that left open a preventable mode of cross-contamination. The case is an appropriate illustration of the importance of improving laboratory practices, but such cases are not proof of known "false reports" commonly resulting from cross-contamination.
- William C. Thompson, The Myth of Infallibility, in Genetic Explanantions: Sense and Nonsense 227 (Sheldon Krimsky & Jeremy Gruber eds. 2013)
- Opinion in the Reference by the Scottish Criminal Cases Review Commission in the Case of Brian Kelly, Appeal Court, High Court of Justiciary, Appeal No. XC458/03, Aug. 6, 2004, http://www.scotcourts.gov.uk/opinions/XC458.html
- "Genetic Justice": The Potential and the Real, June 6, 2011
- There But For the Grace of God ... The “horrific false-positive DNA match”, May 27, 2011
- Have DNA Databases Produced False Convictions?, July 7, 2012
- "Human Error, Bias, and Malfeasance" in DNA Databases and Law Reviews, Dec. 22, 2012
- Good Point, Bad Math: DNA Database Statistics Misunderstood (Again), July 26, 2013