There's nothing more frustrating than thinking you've reached the end of the revision process only to discover you have a big inaccuracy that affects the whole story. Enter yet another round of revising with some rewriting to correct the problem.
The moral of this story? Always fact-check early in the revision process. Even better is to do the research before you ever write the story to make sure you have dates and other information correct. Trust me, it save a major headache later on. The earlier you do your fact-checking and research, the easier it is to deal with revising the manuscript for accuracy. That way you're not having to rewrite pieces of prose you worked so hard to make perfect.
Inaccuracies are always annoying. I'm the first to admit it, since my writing over the years has had enough of them that I've had to correct. Thankfully, I'm getting better about researching before I write, or at least in the early stages of the manuscript, so that my writing is much more accurate than it used to be.
Now, keep in mind I'm talking about fiction here. Non-fiction is a different animal entirely, and accuracy is even more essential. Readers of fiction are a little more forgiving (sometimes, anyway) of miniscule inaccuracies. Any inaccuracy or misinformation in your non-fiction can kill your writing career and any credibility you have. Obviously, you want to do in-depth research before ever starting your non-fiction manuscript. That will save you not only headaches down the line, it could very well save your reputation and writing career.
So, in addition to the million other things you should keep in mind when preparing to write and writing your first draft, you also need to be aware that your work should be accurate. Don't let that scare you, however, and keep you from ever writing a word. Like every other part of writing, you can always revise later, after you've written your first draft and let it sit for a while. Just remember to do another round of fact-checking before you decide your manuscript is ready to submit. You don't want to send it off then realize you got something wrong.
As the title of this post says: Fact-check. Always fact-check.