When I consult with writers, I have to remember "it's fiction." Authors deserve the right to a little literary license. But accuracy and realism are important in crime fiction. Readers appreciate it.
Here's some of my pet peeves in crime fiction:
1. Too many feds. Federal agents make up less than 1% of American law enforcement but monopolize crime fiction. FBI agents rarely work murders, rapes, and other local crimes. Local cops do that. In 36 years of police work, I can recall about five cases besides bank robberies where the FBI joined us in an investigation. Perhaps authors see the feds as more prestigious. I don't know. What I do know is the majority of police work is performed by dedicated local city and county officers and detectives. I'd like to see more of them in starring roles.
Along the same line...
2. Local uniformed cops are relegated to crashing cars and getting shot. Much like in the movies, the star is usually a detective and uniformed patrol officers secondary characters. The death of a uniformed officer may get a paragraph and then the story moves on with the hero detective. Writers don't realize that an assault on an officer brings out all the guns. It's a big deal. It's an attack on our justice system. Everyone from the small-town cop 20 miles down the road to the game warden will respond. The death of an officer is tantamount to losing a brother or sister. It may not be the focus of your story, but don't write it off as no big deal.
3. Weapons. Get the guns right, please. "Pistol" refers to a semi-automatic handgun that uses a "magazine" loaded with cartridges. A "revolver" has a cylinder with chambers to hold five or six cartridges. Revolvers don't eject shells onto the ground. Pistols do. "Mace" is a brand name for a line of chemical weapons. Most officers carry "pepper spray"--a chemical weapon that uses a pepper extract as its effective ingredient. Officers who carry batons are more likely to have the metal expandable version on their belts than the long black ones of years past.
4. By necessity, novels avoid the drudgery of police work--the waiting, the reports, the boredom. But there's no need to ignore the realities. Officers are not running and gunning every moment of the day. It doesn't hurt to mention your hero spent hours writing reports after the big arrest--just a touch of realism to give your readers an accurate picture.
5. Timing. Investigations take time. Evidence analysis takes more time than you can imagine. I've routinely waited six to nine months for DNA analysis because of the backlog at crime labs. That may not fit the timing of your story but don't give in to the tendency to have your characters perform complicated investigative procedures in a matter of minutes. It's not like TV's "CSI."