This is an area that can be huge with consistency problems. Time can get mixed up easily when you are writing a book. Can a character travel from one city to another in the time you have shown in your story? Can all the events tie in along the way? If time is important for your story, you might want to keep a timeline of events so you can find consistency problems. Some stories are told over a school term. Keep the timeline believable. I recently was watching a movie where I picked up on some time consistency problems that made watching the rest of the difficult as I was fixated on the huge time issue. Don’t do that with your readers. Keep them in the story and not confused as to how it could happen in that span of time.
What do your characters look like? Keep the blue eyes blue and the green eyes green. Is the hair long or short? Keep it consistent. Don’t have the girl with a pixie cut in one scene and sporting a ponytail in another, a mistake I made. This confuses the reader and might not seem important at first, but a reader will stop and question the entire story with such inconsistencies.
This includes personalities. If a character has a soft spot for animals, then don’t have them kick a dog out of the way. They have to stay in character though they can develop as a character. Nobody changes overnight. Therefore your characters shouldn’t either.
I like to keep a list of characters and traits at the bottom of my manuscript for easy reference. That helps to keep the characters trait so the reader doesn’t get confused. I might not refer to my characters hair color much, but I need to get it right when I do.
Within a plot, inconsistencies can be found. This includes who does what, when, and where. If something in the plot is off, your reader will get hopelessly lost. This is where it might be difficult as the author will struggle to see issues. Have others read it and see if they find anything out of place plotwise. Authors get too involved in the plot and can see it the way it should be and not the way it is. Outside eyes can spot these problems and help you get them resolved. Don’t hesitate to ask them what they would recommend as again they can see things you cannot.