First of all, courts of law are not places where truth is found. It is a place where reasonable doubt is encouraged, and tie goes to the defendant. It's also a place where, if the jury is not unanimous, the matter is not settled. Using these criteria someone could easily argue that the Earth might well be flat because, if 12 random people were selected, it's entirely possible that the jury would be hung.
However, let's take the standard "forensic evidence" that is offered as the cat's meow, namely fingerprints.
http://www.telegraph.co.uk/news/sci.../Why-your-fingerprints-may-not-be-unique.html
But the basic assumption that everyone has a unique fingerprint from which they can be quickly identified through a computer database is flawed, an expert has claimed.
Mike Silverman, who introduced the first automated fingerprint detection system to the Metropolitan Police, claims that human error, partial prints and false positives mean that fingerprints evidence is not as reliable as is widely believed....
And there are other problems, such as scanning fingerprints of the elderly as their skin loses elasticity and in rare conditions leaves some people with smooth, featureless fingertips....
“No two fingerprints are ever exactly alike in every detail, even two impressions recorded immediately after each other from the same finger....
...there are numerous cases in which innocent people have been wrongly singled out by means of fingerprint evidence.
In 2004, Brandon Mayfield, was wrongly linked to the Madrid train bombings by FBI fingerprint experts in the United States.
Shirley McKie, a Scottish police officer, was wrongly accused of having been at a murder scene in 1997 after a print supposedly matching hers was found near the body....
Previous studies have shown that that experts do not always make the same judgment on whether a print matches a mark at a crime scene, when presented with the same evidence twice.
A study by Southampton University found that two thirds of experts, who were unknowingly given the same sets of prints twice, came to a different conclusion on the second occasion.
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BUT let's suppose that fingerprint evidence really were infallible. What would that accomplish? Not necessarily anything. There could be dozens of reasons for fingerprints found at the site of a crime. For example, imagine that a man needs to use his neighbor's bathroom, asks permission, and flushes the toilet. A few hours later the neighbor is killed. The man's fingerprints are found in the house, his DNA is found in the wastebasket, and he becomes the primary suspect. He may even be convicted based on that alone.
Forensic evidence is really only useful in proving what did NOT happen.