I used a sea turtle in my scuba diving story (Deep Diving; not yet published) and I said that the turtle slowly poked its head out at to look at the characters while they were diving. When I was editing/rewriting, I looked at that and scratched my head. I've seen a few turtles in the water but I hadn't actually seen them poke their head out at me. So I did a Google hunt and found that sea turtles don't have the ability to retract their head or limbs! I'd messed up my land turtles and sea turtles in my writing flurry.
It's those silly little things that can make or break a book for me. If an author gets something wrong that I notice, then I begin to question the book. I worry that I make these types of mistakes because my brain doesn't fact-check when it writes. It only switches on to analytical mode when editing (and it's sooooo mean then—but not infallible).
So fact-checking during editing is important…but sometimes I worry that I invent the facts!