The mechanisms by which flaws enter eyewitness testimony are varied and can be quite subtle.
One way is a person's memory being influenced by things seen or heard after the crime occurred. This distortion is known as the post-event
misinformation effect (Loftus and Palmer, 1974). After a crime occurs and an eyewitness comes forward, law enforcement tries to gather as much information as they can to avoid the influence that may come from the environment, such as the media. Many times when the crime is surrounded by much publicity, an eyewitness may experience source
misattribution. Source misattribution occurs when a witness is incorrect about where or when they have the memory from. If a witness cannot correctly identify the source of their retrieved memory, the witness is seen as not reliable.
While some witnesses see the entirety of a crime happen in front of them, some witness only part of a crime. These witnesses are more likely to experience
confirmation bias. Witness expectations are to blame for the distortion that may come from confirmation bias. For example, Lindholm and Christianson (1998) found that witnesses of a mock crime who did not witness the whole crime, nevertheless testified to what they expected would have happened. These expectations are normally similar across individuals due to the details of the environment.