It turns out that these methods, including fingerprinting, bite mark identification, and ballistics, are not reliable; practitioners testifying in court have little scientific basis for claiming they are accurate. These "experts" have essentially bootstrapped their hunches into accepted testimony by mutually agreeing that their methods work. And on the basis of their testimony, thousands of people have been convicted and some executed.
In addition, some police labs have had to be closed because they were not even running the tests but merely reporting the results that would help convict the person the police had chosen as the perpetrator. These scandals in crime labs involve hundreds of tainted cases handled by police agencies in Michigan, Texas, and West Virginia, and by the Federal Bureau of Investigation. At least 10 wrongly convicted men have been exonerated as a result of those laboratory investigations, and the cases of hundreds of other people convicted with the help of those facilities are under review.