The records also revealed three instances in which DNA samples were accidentally switched, one in which analysts reported incorrect results and three mistakes in computing the statistics used in court to describe the rarity of a DNA profile.
The leading cause of false DNA database matches is cross-contamination of samples, Thompson said.
An incident in a state-run lab in Sacramento illustrated how easily this can happen: DNA discovered on a cigarette matched the profile of a sexual assault victim from another case.
Had the assault victim smoked the cigarette? No. Cross-contamination occurred when the sample from the cigarette was processed close to the victim's vaginal sample.
The risk of DNA contamination has "greatly increased" as scientists have learned how to obtain DNA profiles from a billionth of a gram of genetic material, according to a report last year by the Nuffield Council on Bioethics in London, a group that examines developments in biology and medicine.
"The results may therefore be misleading, and yet they could be presented as powerful evidence in a courtroom. This makes it vital that defendants are not convicted on a DNA match alone," the report said.
Jonathan Jay Koehler, a professor at Arizona State University who has studied lab error, estimated the rate of false DNA matches at about 1 in 1,000, whether they are caught or missed.
"No one would ride on an airline that crashed one out of every 1,000 flights," he said.
Other experts counter that lab errors are rare and will be caught if the proper precautions are taken. "If they are rare enough, then we should be cold-blooded" about isolated injustices, said Charles Brenner, an Oakland-based forensic mathematician.
Jill Spriggs, chief of California's Bureau of Forensic Services, said California's forensic labs are among the best in the nation. "Errors are detected -- if there are any -- based on quality control measures that we have in place."
Wrongful incriminations from DNA evidence have pierced the science's image of infallibility. When Alan Nelson, father of a woman who had been murdered with her daughter in Australia, learned earlier this year that the wrong man had been arrested because of a contamination, he was incredulous: "I thought the DNA was 100% perfect," Nelson told the Herald Sun, an Australian newspaper.
Despite such cases, DNA evidence holds great sway among jurors.
Dan Krane, an Ohio-based DNA expert who has testified in about 75 cases, mostly for the defense, was shaken by the conviction of Gary Leiterman, a Michigan man sentenced in 2005 to life in prison for the nearly four-decade-old murder of a law student.
"It hit me like a ton of bricks," Krane said. "How could that possibly have happened?"
Leiterman, a nurse, came under suspicion when the Michigan state crime lab found a "match" between his genetic profile and evidence from the murder victim's stockings.
But DNA from a blood drop on the victim's hand matched someone else, a man who was 4 at the time of the crime.
It turned out that DNA samples from Leiterman and the other man were being analyzed at the crime lab, as part of separate criminal cases, at about the same time as the murder evidence. Leiterman's DNA was at the lab because of a conviction for prescription fraud. Krane, who testified on Leiterman's behalf, said contamination was the most likely explanation for the findings.