. . . Or “ma’am, just the facts” as Sergeant Joe Friday [actually] said in the 1950s TV show, Dragnet. Whether writing fiction or nonfiction: know your facts. Accuracy is a must.
Devices and gadgets, events and activities, fashion and customs, music and art, phrases and expressions (among others) must be correct for the time/period being written in. Stories are made to entertain. Facts are meant to inform. Exactitude is vital . . . so is [a writer’s] credibility.
If you’re writing a western or historical novel that takes place in the middle of the 18th century, it’s likely people didn’t have tissues or ballpoint pens. Women wouldn’t have worn brassieres and men wouldn’t have known about boxer shorts. One-hundred-and-fifty years ago, it’s improbable that a person would have said, “that’s so cool” except maybe if referring to the weather. They’d not have said “sweet” unless commenting about a dessert or fruit. Become familiar with the period of time being written in.
If you’re writing a story that takes place present day and are using real places, ensure the details are accurate/correct. Don’t mention that John went to a concert at Massey Hall in Toronto and have it located on the other side of town. A real place should be in its actual location. If your story is set in a city or country you’ve never been to, acquaint yourself with it. Anything related to the here and now—and the story—should be properly (accurately) detailed. Quoting someone? Sure, have at it, but ensure the quote is correct.
That holds true of any genre, including fantasy and sci-fi. Granted, you may be able to stretch some truths, given these worlds don’t [yet] exist 😉 but you’ll likely be incorporating some technical or scientific details. You may even refer to events or inventions that lead to the creation of your future/other world, so it never hurts to become familiar with technology or science. Research is never a waste of time (at the very least, you’ve learned something new).
The example above—“ma’am, just the facts”—is how Friday said it and not the way we often use it or see it: “the facts and nothing but the facts”. This brings us to something that you may want to do and not leave to an editor, who may not always cast that critical an eye: fact-check. This process verifies that information is factual and ensures the story/writing is correct and concise.
You can fact-check as you go along in your writing or do it at the end of the first/last draft; determine what works best for you. My process is that I’ll write a scene, edit it, and note what I’d like to expand on, like a setting or dwelling, clothing, whatever. Say one of my characters is attending a luau. If I want readers to get a taste of what that entails, I’ll research luaus—preparations required, types of food and entertainment, locations (where might they take place), and so forth. I may have read pages (!) of details but, in the end, only write a couple of paragraphs. But that piece of writing will be descriptive . . . and accurate. 😉
Get facts straight. For all intents and purposes, your fictional world is the real world to a reader. Don’t disappoint them by having glaring errors. And don’t disappoint yourself by not having done [provided] the best [most accurate] work that you could have.
The Writer's Grab-Bag 
What you say is very true. Along those lines, it is alarming that many people in this country do not want full truths about history, and other subjects, taught in schools. Those warped policies have been instituted in some communities and states in the USA.
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Yes, that is so [unfortunately] true. Historical and current facts should never be altered to fulfill a [corrupt/self-serving] goal.
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