As of yet, I have not had to face down a policeman claiming to be a gang expert (until recently local police denied any gang presence). However, I can entirely understand the sentiment. Personally, the "experts" which bother me the most are police "drug experts."
What, you ask, is a police drug expert? Well, an example of one I faced in court was a 7 year veteran of the force who had spent the last 5 in the street drug task force. Only a couple problems with that. First, he was quite clearly biased. Second, he had absolutely no training beyond street experience and whatever internal training the police offered. He had no objective training, no exposure to any sort of research or statistical data which could indicate probabilities based on activities or the presence of certain items. Basically, he was there to testify that my client was in some way similar to others whom he had arrested.
His testimony was that my client had to be a dealer rather than a user because he had a cellular phone and a beeper, he had a couple hundred dollars, and there were plastic bags under the driver's seat in his mother's vehicle (which he was driving). The drugs (one rock and a decent amount of marijuana) were in a bag which my client claimed he did not know was in the back of the van. We were able to put his boss on to testify that the money was from getting paid that day in cash (construction workers are often paid under the table with cash). The mother, rather indignantly, testified that the bags were hers and she always kept a box of them under her seat because she had grandkids and put opened candy, rattles, pacifiers, etc. in the bags so they wouldn't get dirty and wouldn't screw up her vehicle. As for the beeper and cell phone, when the police took the phone they also took its charger - leading to this exchange:
Me: Officer you see this item? What is it?A couple of jurors actually laughed at the guy. I had expected the officer to say something about how even the cheap phones have to be recharged and planned on pulling out the phone because it was obviously not a throw away phone. But I didn't think it could get much better than that so I left it alone.
Officer: A phone charger.
Me: That would indicate that the phone is a permanent phone - not one of those you buy with 100 hours and then throw away?
Officer (looking puzzled): Yes sir.
Me: The point of the beeper/phone setup for a drug dealer is to have a beeper to get called on and a phone which cannot be traced back to him to call back on?
Officer: Yes sir.
Me: So, that doesn't work if you have a permanent phone does it?
Officer: No sir, but it's my experience that criminals aren't that smart.
Of course, this sort of thing works in a jury trial. When you do it during a bench trial the judge compliments you on your cleverness (at least they do around here) and convicts your client anyway. I sometimes think that police "experts" could tell a judge the sky was green on the day of the offense and be believed.
On a side note, I still think I would have won the trial if it hadn't been for the blowjob coupon book.