Your hero has cropped, curly, dark hair—wake up call to the reader when you write "…his long, blonde wafted in the breeze…" and destroy the image within the reader's mind.
Or if you claim "She stood on the castle wall and gazed at the lake at the edge of the fields." and then later the character rides her horse to the lake and it takes three (3) days! Just how big were those fields?
This is where the writer should have cheat sheets!
If you are writing a tale that involves great distances and/or several locales—Make a map. If you're writing a science fiction epic, depict the separate star systems on individual sheets that 'could be' placed side by side on a floor to show the universe of your mind's story. J. R. R. Tolkien may have started his "Lord of the Rings" by the seat of his pants but I'm sure before he ended, that map of Middle Earth came into existence early to help keep him on the straight and narrow.
Each character, especially the main/lead characters, should have individual description pages detailing them. This will help you, the author, keep straight whether your heroine has short hair and brown eyes or long hair and blue eyes. The character development page can be as simple or complicated as you need. I've used one for several years that is very intense but only fill in what I need. I've also discovered that picking an image from the internet, be it a star, starlet, dignitary, or even just a random face—it helps me see the character better. I use those plastic protector pages and put my character description on one side and on the opposite side, an image. It helps to keep me organized and works especially well for recalling the characters when doing a series.
Even a house or building needs to be accurately described. Imagine a bathroom with three doors! Yes, I've seen one and for me, the idea of privacy and use did not come together very well as a possibility. If the author does nothing more than an obscure drawing of circles to designate the rooms, it will help with the flow of the story. I once created a two-story farmhouse with stairs both in the front and in the back. I had the back stairs at one time to the right of the backdoor and the next time coming down near the kitchen stove on the opposite wall. It was then I realized I needed to "visualize" my house and get it on paper.
Even after the story, whether it be a short story, a novella or even a nice sized novel is completed—it needs to be evaluated. There are ways to do this through various check lists.