As I complete my final edits of my book I think one of the most challenging things that I've had to deal with is the aspect of fact checking.
It's humbling to write a book when dealing with aspects like these, and you will find yourself feeling demoralized when you go back on the final round of editing and find out that Character A gave a sword to Character B and you completely forgot about that. Generally there's a few ways to avoid things like this, and here I've compiled some methods that can help you get a good handle on the nitty-gritty in your novel.
Writing it down...
Maybe not terribly ground breaking, but it helps to just write things down.
What I do here is that I start at the top and work my way down, and at each successive layer the imagery gets more and more intensive. Recently I just began work on a comic book, and at the very beginning I drew the map of the world, or at least a rough approximation of where the story was going to occur. Then I documented the names of the "leading" figures in each location. From there I wrote down what the name of their spouse was, their kids, their ages, and any pertinent background information. I do this so that when I'm two hundred pages into the book and character A says that he came to the town at a certain age, I could approximate where that character stood in relation to those around him. In one instance, a certain character comes to my primary location when he's ten, which means that another character in the book was in his early twenties, which gives me a handle on what kind of back story and potential sequels I could work with. About 4 years ago I created a document full of information like this. I still use it today, and it is very, very helpful.
Who has the Sword?
Always ask yourself in any scene you are writing, "who has the sword?" It's a rhetorical question, obviously. Think about it like this:
There are three characters in a room. Character A is sitting, Character B is crouching, Character C is smoking. Here's the question. Who is standing?
It's a trick question. None of them are. Until you move your character in a narrative think of them as static and immovable You will see this in movies. Take a look at any scene in a movie where the entire ensemble cast is on the same set. Generally the only people that move and react are those that are speaking their lines, otherwise everyone else is still. So it's important to remember the placing of your characters. Saying that Character A is sitting will wash over subconsciously in the reader, but later on in the dialogue, if Character A is suddenly standing the reader will know something is wrong. The entry of detail and the transitioning of character states will create the image of movement and dynamics in a scene. Therefore, keep track of "who has the sword."
I will add here a little tertiary detail, that the static characters, while "silent" are assumed to be paying attention to what is going on. If I wrote a small dialogue with these three characters and said, "Character A looked up at Character B and asked, 'how's the weather?'" it will be assumed that Character C watches this happen. Even in our minds you can watch the line play over and over fluidly. Character C is "smoking," looking on while the others exchange their dialogue. If you want static character to do something in the dialogue, you need to say it up front before the dialogue happens, otherwise the reader will be wondering if they just started doing this or if they had been doing this prior.
Again, these are things that have helped me in fact checking in a book. The human brain is very intricate and powerful. Don't sell it short! Subconsciously, it can absorb torrents of information, so it remembers details from long ago in a narrative. When inconsistencies come up, it's very jarring, so take into account what your characters are doing at all times. It's difficult to get used to, and takes time to figure out, but when it all clicks, your story gets much better.