Eyewitness identifications are often believed to be among the strongest evidence possible. If someone says they saw the defendant commit the crime, chances are most people will believe the defendant is guilty.
Eyewitness identifications are also a common point of drama in crime stories. A witness looks the defendant in the eye, points from the witness stand, and says, "He's the one. I'm sure of it."
However, many studies have found that eyewitness testimony is notoriously untrustworthy. Of 250 exonerations based on DNA evidence, 190 prisoners had been convicted based on mistaken eyewitness identifications.
That said, identifications are generally only kept out of court when the police used unduly suggestive tactics to get the witness to identify a particular person. Otherwise, traditional trial protections, such as cross-examination, expert testimony, and jury instructions, are relied on to protect against a jury wrongfully convicting someone based on a faulty eyewitness identification.
Whether that will continue to be the case will be decided soon by the U.S. Supreme Court in Perry v. New Hampshire.