There are several words and spellings that people are always asking about. Grey or gray? Color of colour? Toward or towards?

And very often the truth is that it doesn't much matter. What matters is that you are consistent.

If you know it's a word you inadvertently switch around or your word processor changes or something, once you're done, just search for the word. Check out each use. Change (or find and replace) the rest. Don't forget about plurals and conjugations (hence why I like to ctrl+f and look at each use of the root word).

In dialogue and to some extent inner monologue, characters acquire their own voice. It's alright for a character to use a variation that's different from the narrator (such as using a Norwegian word in place of an English one or a character using broken English)! However, if all that's different is the spelling (grey vs gray), keep it consistent with the narrator. Your British lady doesn't need to use British spelling to be British (and if she does, you didn't do your job setting up her character!). She might, however, call a truck a lorry, and that is 100% OK!

Spelling of names.

John vs Jon vs Jhon, Brian vs Brain (I used to write this one all the time to a friend of mine!)

Capitalization.

Kayla, kayla, Duke of York, duke of York, etc.

There are some title rules! Logan is a prince. You can marry Prince Logan or a prince from a faraway land. As a general statement, a specific person should have a Capital, nonspecifics don't need caps.

Spelling of nouns, verbs, etc.

Toward/towards, grey/gray, etc.

British English vs American vs whatever language you're writing in.

Don't pull the British punctuation and quotes and then go 50-50 on spellings, etc; depending on your editor you're going to see different corrections.

Character traits such as eye color.

If you don't remember, write it down. Most people get the MC consistent, but the secondary characters tend to suffer from forgetfulness because they aren't as important.

These are the basics. The things that are easy fixes and not that hard to notice in your rough/first/second/etc draft. There are other things that are important, but they can be more difficult to track and changes might require more than subbing out a letter or word. I'll list 'em though, if you're in that polishing stage where you want to make sure things are perfect.

Character traits such as personality, likes, dislikes, allergies.

If you give your character a cinnamon allergy and ten chapters later she's drinking Mexican hot chocolate, there's a problem.

But I catch a lot of issues with fears, shyness, and stubbornness. I know I touched on this in an earlier chapter (Flaws), but if your character is told to have a trait, make sure it actually shows up in the story!

Character inventory, within and outside of scenes (especially hands!)

Especially in battle scenes, I've noticed that people tend to forget what characters have and don't have. Two chapters ago they pulled a sword from the stone or found a gun under the bed, and now a zombie's charging them and they're like: OMG!!!!

That, or sometimes characters are holding so many things they must have nine arms. Sometimes people have to put things down before they can whip out their bow and arrow!

unless you're early in the first chapters, your reader should be aware of your character's inventory. It shouldn't be a surprise in chapter 27 that they've got an extra knife hidden in their waistband all along (unless you eluded to it in some way earlier)! It's okay to keep secrets from your readers, but you've got to tell them that there is a secret, otherwise it's just a "Well, isn't THAT magically convenient!" moment.

Time

You can pass time in many ways and I'll have a chapter on that later, but please make sure that your time is consistent with how long an action might reasonably take. Here are some examples of poor time management:

Mike lives an hour away by car. He receives a frantic call from his girlfriend saying she's walking home and some guys are following her and she thinks they're planning to assault her. Instead of calling the cops, Mike arrives on scene fifteen minutes later.

Hillary writes a ten page paper on the chemical properties of soap in a under an hour. Automatic A. *sigh* School is so easy! [Coming from college I've seen how fast some people can pull papers out of their butts, still, this doesn't usually translate well in stories unless it's exactly that kind of situation.]

Sarah goes to bed at 10 pm. She hears a knock on her window at midnight. It's Dixon. He warns her that the Weres are after her. And then he has to go, because dawn is on the horizon.

Alexei has to walk 50 miles to reach the Dragon's nest. She begins in the morning and arrives shortly after nightfall.

Abilities

If your character has special abilities, especially inhuman ones, be consistent about their usage. Don't make them magically forget not to use it so it suits your plot! If there's a situation where they cannot use them, make sure it's clear why, otherwise it'll seem like it's just stupid or forced.

Lois Lane is trapped in a bank vault. Only the villain has the key. Superman suddenly doesn't just rip open the door. [because he's Clark Kent today, or it's laced with krypton]

Harry has x-ray vision. Someone on the train is wearing a bomb.

Agent 49 is the villain's right hand man. He's defeated countless enemies and has superior ninja assassin skills. The teenage heroine recently thrust into this spy world knows he's sneaking up on her.

As long as you are reasonable and aware of character's abilities, you can write a convincing scene.

Friends, family, and foes

Disappearing characters are necessary and sometimes good. You can't have everyone in every scene. Busy characters, characters who have lives outside of their interaction with the MC, are even better. They tend to be better developed, too!

If you're going to have side characters, consistently give them reasons for not appearing. People don't need every detail, but you don't want them asking things like: But what about Sargent Mills? He was such an important character and he was just gone by chapter 10. Makes no sense.

Why weren't Mulan's parents also in the army? Why didn't they go after her? Only one person from each family was called. Discovered women were killed. Done.

Our hero goes everywhere with BFF, but she attends the school dance alone. She says, "Allie couldn't come so I was flying solo tonight."

Well, why? Our hero's BFF has surgery for her torn ACL. Easy.