If the police tell you they have an eyewitness willing to testify against you, it is essential not to give up hope. Remember that the police and prosecutors may use eyewitnesses to pressure you to accept a plea deal. Even when they know the eyewitness is not that reliable.
The Innocence Project puts misidentification by eyewitnesses as a leading cause of wrong prosecutions. To understand why first imagine you are the investigating police team and then the jury or judge.
Eyewitnesses solve the case for the police
You and your police team are getting nowhere. Then you find someone who claims to have seen a person at the scene of the crime. It will come as a huge relief. Your team can switch from searching for clues to focus on finding evidence to confirm that theory. That subtle switch of direction can cause you to overlook or dismiss anything that does not support that theory.
Eyewitnesses make it easy for the judge and jury
Working out whether or not someone is guilty is challenging, especially if you are not a trained professional and merely someone on jury duty. Yet, here is someone standing up in court, taking an oath and saying the person in the dock did it. Considering the consequences of lying under oath, it seems unlikely that anyone would give up their time to stand there and say that for any other reason than it is true. Why doubt them?
The problem is that eyewitnesses often get it wrong. Their eyes and memory are not as reliable as they think. If you are accused of a crime, there are many ways to challenge the charge and challenge an eyewitness testimony. Understanding them will be key to your defense.