It is pretty well accepted among most lawyers that eye witness testimony is often inaccurate.  The story I heard in law school was that a professor staged a fake attack during class.  He had someone walk into his room and pretend to shoot him.  The shooter ran away and the professor jumped up to show his good health.  To the students, it looked very real.  The professor then asked the class to describe the shooter.  The professor got some 30 different descriptions.  It is very hard to describe a person or an incident accurately after just a few moments.  

Now, we know this even more so, since so many inmates on death row have been freed with DNA evidence.  In a recent Scientific American story, they report about a supposed rapist and killer convicted based on five eye witnesses.  He too was freed with DNA evidence.  73 of the 239 death sentences overturned with DNA evidence relied on eye witness testimony.  One-third of those 73 cases relied on one or more mistaken eye witnesses.  

The authors of the article explain that one popular misconception about the brain assumes we see an event and later, we simply re-play that same event.  Not so, say the researchers.  In reality, we reconstruct the event, much like putting a puzzle together.  Later, questioning by a lawyer may alter the memory by causing the witness to confuse actual recollection with information provided by the questioner.  Researchers have succeeded in creating false memories in various studies, simply by suggesting realistic, but inaccurate facts.  One-third of the subjects in one study reported recalling partially or completely the false information.  

So, the article explains, various factors can cause faulty memory:

• extreme stress at a crime scene
• presence of weapons at the scene of the crime (adding to the stress and causing distraction)
• racial disparity between the witness and the suspect
• brief viewing times ate a lineup or other identification procedure
• a lack of distinctive characteristics, eg a tattoo