Not only did PTPD bring up the obvious, there is one more. Lying on the stand can "Brady List" the officer. This effectively discredits ANY of the officer’s future testimony pure garbage. Very few counties will allow a "Brady" officer to testify. This effectively ends the officer’s career, being their word is worthless.
I can't even tell you how many times people say they adamantly say "I wasn't speeding", "I stopped for that stop sign.", "I wasn't on the phone.", "You sure it wasn't the other guy?", or "These aren't my pants officer, I've never seen those drugs.". In this situation, the officer witnessed the violation and stopped said violator who was now walking away. The officer stopped the guy and questioned them, then determined they were DUI. It's simple. People lie to cops almost ALL the time, even the "normal" people of society who have never done anything* wrong in their life (*never been caught). If cops took someone’s "word" for something all the time you would have a really gullible police force.
I'm a cop. My opinion does not reflect the views or opinion of my agency. That being said, "Do not argue with an idiot. He will drag you down to his level and beat you with experience."