I'm currently reading a sci-fi book that has over a dozen characters. The author had written dialog for two different characters using the same odd idiom in two separate chapters so far. I cannot see this being intentional, and to me, it is visible and easily detectable by writing software like Scrivener.

I've noticed this sort of thing in multiple novels. So, why do novels end up with such mistakes? Aren't there proofreaders, editors, publishers, etc. who review the work before publishing? What precautions should writers take, not to fall in such mistakes?

Lack of proofreading has been the bane of writing in many locations over the last few years.

Do you remember back when newspapers came to your house and you paid to subscribe? Okay, maybe you don't, due to age or location, but it was a thing. Most people (at least among the college-educated folks I knew) subscribed to the daily local paper which was filled with articles, columns, and all sorts of written things.

Those newspapers had a little section, usually on or near the editorial page, that noted the mistakes from previous issues (rarely more than 1-2 days old). Factual errors (saying 14250 tons instead of 1405 tons) of course were top of the list. But they also noted regular mistakes. Misspellings of names and such.

As for ordinary typos, they were rare. I only sometimes found a single one in an entire issue. Ditto for magazines. And published books? hardly ever.

But now people get their news online, read only selected magazine articles sent to them in email or on the company's website, and download e-books for free or 99 cents. For some of these media, subscriber fees were just part of revenue. But now ads pay less as well. As recently as 5-10 years ago, a good blogger could get a decent income (not a living, but a good supplement) from hosting ads. Now the ads pay a fraction of what they used to.

All these means is there is less staff available to check manuscripts. It's endemic in online columns. Popular and profitable columns like Ask a Manager and Savage Love have 1, 2, or 3 typos and other obvious errors per column. These are not blogs and they have paid staff.

But you didn't ask about periodicals. You asked about novels. They are connected though. Less money in means fewer staff people. And the whole self-publishing trend has changed the industry. Whether it's blogs or novels, people do it themselves and they don't always think a professional editor is important.

How can a writer protect against this?

First, you have to care. You do. I do. But, frankly, a lot of people just don't. They don't notice the mistakes or they just don't think they matter. (And when they're in an environment where mistakes matter greatly, they are appalled and try to escape.)

If putting out a perfect manuscript is important to you, you'll budget the time and expense to make it happen.

Second, you have to oversee. If you have a traditional publisher, you have to check that they're doing what they should. If it's a top publisher, they probably are. But smaller presses might cut corners. If you are self-publishing, you have to make sure the work happens and then oversee it to make sure it's being done right.

Every novel needs several stages of editing. I can't list them all off but, if I were self-publishing, I'd find the lists online and in books and study them and create a version that worked for me.

What most self-publishers don't understand is that editors can't be the author and that they can't be amateurs. Sure, use beta readers and friends and family, but that's not for pre-publication editing, it's to get the manuscript to a stage where it's ready to submit to publishers. The editing comes after acceptance.

Pay for the work.

If you have professional standards for your work (and we all should!!), then hire professionals. An editor to work on structure and the big picture. Editors for scenes, dialogue, and smaller things. A proofreader. And someone to do the formatting setup for e-books and/or printing. One editor might be enough and some editors can also proofread. But the process takes multiple steps and can't be done all at once. Not for something large like a novel.

Third, do the final read yourself.

May all our works be perfection!

It could be that the author is highlighting the similarities between the two. It could be creative provincialism (A U.S. writer not knowing that a British person would not say that, or vice versa). It could be that strange minds think alike (An example from Spongebob Squarepants where both Patrick and Mermaidman independantly believe "Wumba" is the opposite of "Miniturize" or in Archer, where upon learning that the situation involves the Prime Minister of Italy, several characters independently offer up they though Italy was still a Monarchy with the same reaction of "Wait, I thought Italy has a King.")

Another example is that it's an early installment weirdness, where the series is in it's infancy and still trying to find it's voice and proposes concepts and ideas that are later excised and made impossible. It could even be that authors and writers have styles that can be observed with increased familiarity with their works. Joss Whedon, for example, has a reputation for killing off innocent and fan beloved characters in horrible ways. Greg Weisman (who does a lot of Cartoon Works) has a fondness to Shakespeare References and villains' who kick off the episode plot to distract from their real goals.

I just recently got an ARC from an author who's a friend of mine. Reading through it, I noticed that in one scene, a character started out drinking one thing, but at the end of the scene there was something completely different in their cup. I mentioned the continuity mistake to the author, and the response was basically "Oops, I started with the one thing, then changed it because I realized it was out of character, but I missed changing it in that one place. Good catch!"

So, why do novels end up with such mistakes?

Because people miss things. In this particular case, the author mentioned that their usual proofreader had been unavailable due to a health emergency.

Aren't there proofreaders, editors, publishers, etc. who review the work before publishing?

Sometimes. Sometimes they aren't available. Sometimes the author just doesn't hire one at all. In this age of Kindle self-publishing, you unfortunately see a lot of "shovelware" books that are basically a first or second draft dumped straight onto the Kindle store because it's quicker that way.

What precautions should writers take, not to fall in such mistakes?

There's a principle in computer programming known as "Linus's Law," after Linus Torvalds, the creator of Linux, who relies on this heavily. The law states "with enough eyes, all bugs are shallow [ie. easy to find.]" The same principle can be applied to writing. Show it to people before you publish. Not just the editor, not just the proofreader. Show it to friends. Show it to your writing group. (Get a writing group if you don't have one!) Show it to readers who have sent in insightful feedback on your previous books. (That's how I ended up in a position to get this ARC.) Get enough different people looking at it before publication, and you'll end up publishing something a lot more polished than you would otherwise.