An investigator who does not know the limits of a technique will believe a result that the technique cannot support, and will stop looking.
A result is a comparison of propositions, not a verdict. DNA analysis says how much more likely the observations are if the sample came from this person than if it came from an unrelated person. It does not say the person was there, when they were there, or what they did. The step from "their DNA is on the handle" to "they used it" is an inference somebody has to argue, and it is where cases are lost.
Transfer and persistence. Material moves. DNA can arrive on an object through an intermediate person or surface, without its owner ever touching it, and it can persist for a long time on some surfaces and vanish quickly from others. The presence of a trace is not a record of an event.
Disciplines are not equally well founded. The 2009 report of the United States National Research Council examined the forensic disciplines and found that, apart from nuclear DNA analysis, many pattern-comparison methods had never been validated to the standard their courtroom language implied. Bite mark comparison and microscopic hair comparison have been the subject of particular criticism and of overturned convictions. The word "match", used without a statement of how often such a correspondence occurs by chance, claims more than the method can support.
The examiner is a person. Knowing what answer the investigation wants measurably shifts interpretation. Good laboratories manage this deliberately — by giving the examiner only the information the comparison requires, and in the order it is required. An investigator who telephones a laboratory to say "we are sure it is him" has contaminated the result as surely as if they had contaminated the sample.
What to do with all of this.
- Ask what the result would look like if your suspicion were wrong.
- Ask what the number means, and if nobody can tell you, treat the result as an indication rather than a conclusion.
- Never build the case on one comparison and stop.
- Write "consistent with" only where you can also write what else it is consistent with.
Source. National Research Council, Strengthening Forensic Science in the United States: A Path Forward (2009), https://nap.nationalacademies.org/catalog/12589/strengthening-forensic-science-in-the-united-states-a-path-forward ; National Institute of Standards and Technology, forensic science, https://www.nist.gov/topics/forensic-science and the Organization of Scientific Area Committees for Forensic Science, https://www.nist.gov/osac
What this is not. A basis for challenging or defending any particular result. That is done by qualified experts on the material in the case.