One of the reasons why it is so detrimental for members of the New York Police Department to be charged with a crime is because it makes all future testimony suspect. It is much harder to take an officer's word as true if he or she has been convicted of a crime, at least that is how many people in the criminal justice system see it. Just because an officer has a clean criminal record, however, doesn't mean that he or she can't lie under oath.
Since most officers are trusted and their testimony carries great weight, judges and jury members are likely to believe what officers say. What happens when an officer is lying about a suspect? That suspect could be convicted of a crime that he or she never committed.