The New York Times has an article detailing how DNA can go wrong; the title is High Tech High Risk Forensics.
Accurate DNA results depend on several things. For starters, the evidence has to arrive at the lab in the same condition it was in when collected. The evidence can be contaminated if not properly handled; the story in the article is about paramedics who transferred evidence from the defendant to the crime scene.
Another area is statistics. DNA experts typically talked about possibilities of finding someone else with the same markers, and the numbers are often astronimical. Those calculations are often taken for granted - which they shoudn't be. The basis for the statistics may not be proper, or the calculations may not be performed correctly.
All of this reinforces what we have recognized for several years. While DNA is set forth as the gold standard of forensic evidence, it's not infallible. You cannot take it for granted, and must check and verify everything.