All writers make their own share of mistakes, and we all make them in our own special ways. There are, however, some mistakes that regularly crop up – not just in the work of new writers, but also writers who have some experience under their belt.

Dull writing. This can be hard to define, not least because different people like different styles of writing, but it’s a common mistake to see writing that just seems lifeless. Being grammatically correct and getting the mechanics right is one thing, but writing in a way that is engaging and interesting is something else entirely.
Too much unnecessary detail. Not every sentence of your novel will move the plot on. Not every sentence will add anything to the development of your characters. However, too many sentences that don’t add anything to your novel is a big mistake, and it’s one that writers often make. It’s easily done. Often when we want to see our word count increasing, it can be all too easy to just write anything – even if it doesn’t really do anything for the story – telling ourselves we’ll edit it out later, only for it to be left. Cutting words out of your novel can be hard, but it’s important – be ruthless in editing.
Lack of pace. The pace of your novel won’t always be consistent – sometimes you might spend several pages on a relatively small moment, while at other times things might rattle along at a fair old lick. That’s fine – you have to go with what the story demands. However, a story without any pace, or a pace that doesn’t make sense for the tale you’re trying to tell, definitely counts as a mistake.
Lack of structure. When you write the first draft of your novel, it might not have much of a structure as you’re still trying to sort out the story. However, as you progress with the writing process, a structure should start to emerge. It might not be obvious, but it ought to be there; you should be to identify the key points of the beginning, middle and end.