I was grateful when a few ex-law enforcement authors on here replied to one of my posts and pointed out that my premise for a police procedural was flawed. I was thirty thousand words into the WIP and it knocked me for six. I put it to one side, thinking all was lost and moved onto other WIP stories I had on the boil. Thanks to their input, and more research, I've gone back to it and done a re-write to bring it within the realms of procedure for the LAPD/LASO area. If I hadn't done that, when I eventually complete the story and publish, I could have fallen foul of readers for the genre who take it seriously and probably they would have given it a 1 star. Police procedural stories have at their core the same sort of dedicated readers that the harcore sci-fi genre has, and they will be quick to pick up on a poorly researched story.
Sometimes you will say come across such as - "He withdrew his Glock 17 and flicked the safety off with his thumb", when in fact the safety for that is part of the trigger mechanism, like a second trigger that you pull with your trigger finger first to enable the Glock to fire. It's those sort of things that can turn a reader of the genre off from continuing if you get it wrong.
Like others have said, research is important for a police procedural. If you can't find someone in law enforcement to talk to, there's plenty of information on the web, including official sites, together with sites that list different laws for different states. It's okay for cops to break the rules in procedure, and the law, because some do. But it helps if you know when they are doing so.