I highly recommend everyone who has not read this article to do so:
http://www.newyorker.com/reporting/2009/09/07/090907fa_fact_grann
It exhibits in detail almost everything being discussed in this thread.
A professed "expert" who wanders around a crime scene like Adrian Monk sets up a completely biased framework by announcing a conclusion. Then all of the eyewitnesses subtly change their stories (not intentionally) to match this established understanding of events. The poor gentleman is convicted because of horrible science and the fact that he was behaving strangely and appeared to be guilty.
*Warning, Opinion* As a lawyer and someone who has studied very closely a number of bad death penalty cases, I am almost at the point where I think all eyewitness testimony should be disallowed in court cases. That fuzzy science stuff is a close second, but those issues tend to be dealt with naturally--better techniques supplant outdated ones, except in Texas, evidently.
In any case, the way to think about the issue, as with all due process/legal system questions, is to imagine yourself in the place of the defendant. NO ONE would argue in favor of pupil dilation analysis or body language experts or whatever hocus pocus happened to be popular at the time if their ass was on the line.