"I have noticed that nothing I never said ever did me any harm."
Impeachment by Omission
Defense attorneys know that if they can get a judge or jury to doubt a police officer, it can cause them to doubt the entire case. That's why they attack you more ferociously and often than other witnesses. One of the ways they do this is by trying to impeach your credibility with some fact you testified to at trial but omitted from your police report or in your prior testimony, e.g., in a search warrant proceeding, at grand jury, in a deposition, or a suppression hearing.
Let's look at an example.
You get a call from Rafael Alujas, an acquaintance of Varas. Alujas tells you that Varas is going to Ginard's home to pick up some cocaine. You and a DEA Special Agent set up surveillance. Soon, you see Varas drive up to Ginard's house, go inside for a few minutes, and come out carrying a brown shopping bag. Varas gets in his car. After he drives about 100 yards, you and the Agent stop him. You both identify yourselves and observe the bag in the middle of the floorboard within arm's reach of Varas' right leg. You ask his permission to search the car and he says, "No problem." You ask him what's in the bag and he doesn't respond. Under crumpled floral paper at the top of the bag, you find a brick of cocaine. Ginard's print is lifted from the brick, but none matching Varas. Both men are arrested.
At trial, Varas testifies he didn't know the bag contained cocaine. He had agreed to pick up what he thought was a power drill at the request of Alujas - a mutual friend of his and Ginard's. The defense theory is that Alujas set Varas up to get a break in his own pending criminal case in exchange for providing information about the "criminal wrongdoing of other persons." Alujas gets 3 of the 4 counts against him dismissed. The only information he provided the government was the tip against Varas.
You testify at trial also. You state on direct examination that when Varas was asked whether his car could be searched he appeared nervous and he appeared more nervous when asked about the bag. [See the link below for Bullet Proof Your Credibility in Court and read why you should NOT put "appeared nervous" in your police report.] You did not mention Varas' nervousness in your report, at your pre-trial deposition or when you testified at the suppression hearing the day before trial.
The defense will cross examine you something like this:
Officer, you prepared a police report in this matter, correct?
How long after you observed Mr. Varas and arrested him was it before you wrote your report?
You're specially trained in how to write police reports, aren't you officer?
That training begins in the Academy?
It continues through your field training?
And it continues on the job?
You know your reports will be reviewed and signed off by a supervisor?
You're specially trained and take care to write reports that are thorough, correct?
And a thorough report includes ALL the relevant facts, doesn't it?
You rely on your police report to refresh your recollection before testifying, don't you?
Why? [Because a significant amount of time will have passed and you can't be expected to recall the details of each encounter without reviewing your report.]
You testified when the prosecutor was asking the questions that Mr. Varas appeared nervous, correct?
Show us where it says that in your report.
It's not there, is it officer?
Show us where you testified to that at your pre-trial deposition or the suppression hearing in court just yesterday?
You didn't mention it then either, did you?
Yet now, 6 months after the event you're testifying for the first time to this additional detail?
How many people have you observed and approached on the job in the last 6 months?
You know the government has to prove beyond a reasonable doubt that Mr. Varas KNEW there was cocaine in the bag, don't you officer?