That's true of any witness, though. Perjury is a crime, but you have to prove it.
In the trial context, good defense attorneys will be able to introduce contradictory evidence, or will challenge the credibility of a witness by finding facts that make them look bad -- even police officers. With police officers it's a bit harder, though, because the public largely believes (wrongly, I'm afraid) that police are more trustworthy than the general public.
In the trial context, good defense attorneys will be able to introduce contradictory evidence, or will challenge the credibility of a witness by finding facts that make them look bad -- even police officers. With police officers it's a bit harder, though, because the public largely believes (wrongly, I'm afraid) that police are more trustworthy than the general public.