Interesting comments in Henry Lee's book "The Real World of a Forensic Scientist"...
pg 293 discusses Joyce Gilchrist, a technician that testified about hairs in such a way as to imply that her microscopic comparison was a "match"...
Lee: "Even though every well trained forensic scientist knows that hairs can only be "similar" and never associated with one and only one source."
In a previous case where she testified, the case was overturned because the state supreme court said her hair testimony was improper and outside the standards of forensic science.