Those of us who deal with them know that crime labs are not infallible. That’s because they are staffed with people. Here is an ongoing issue that’s coming to some intermediate resolution.
Remember Brooks AF Drug Lab (and how they initially hid the problem). http://www.innocenceproject.org/Content/Gross_Misconduct_at_Military_Crime_Lab.php
Remember the Mobley letter, an issue of bias and how military prosecutors might complain when their own experts testify truthfully.