In criminal cases, eyewitness identification is notoriously prone to error, especially when the authorities overseeing the process fail to follow basic procedures to ensure that witnesses aren't improperly influenced.
One scenario that should always prompt an aggressive challenge from D.C. criminal defense attorneys is that of a "field show-up." This is a situation in which, immediately subsequent to an alleged crime, a witness is brought to the scene where the suspect has been detained and asked, "Is this the guy?"
The problem is that if the suspect is handcuffed and/or in the back of a squad car at the time of this encounter, this creates the suggestion of guilt. This can be particularly problematic if the witness has yet to provide a full description of the suspect to the officers prior to that encounter. It's also considered an issue if only one possible suspect is presented to a witness, again, creating a higher suggestion of guilt.