Lying
Once you start lying, you're on the road to becoming a Brady Cop. I'm sure your academy taught you well about the cases of Brady v. Maryland, 373 U.S. 83 (1963) and Giglio v. United States, 405 U.S. 150 (1972). If not, call up your prosecutor's office and have them give you their spin on the matter.
In a nutshell, if you have lied in your police career and it is known, the details will have to be disclosed at all trials. There are entire books on criminal procedure written about this; it is that important.
If you are a known liar, your word is worthless; your truth and veracity is no more. To most, you have been eviscerated as a cop. Be truthful in all actions, reports, testimony, and court actions, including any dealings with evidence. All of these are important; they all matter.
I recently read of one department where two of their officers' cases are no longer accepted by the federal prosecutor's office due to this. In some states, this can be a reason for dismissal for you can no longer properly perform your duties as a police officer.