For a book with as thick a plot as a novel, it is easy to make mistakes. Most of these are hard to notice and bear no great effect to the work. However, some of these are easily avoidable and could be damaging if left unsolved.
Forcing plot development
Various factors, such as a deadline, could force you to rush the work. You may therefore, as a result, force plot development instead of gradually developing it. This creates an immature art piece with a lot of unresolved issues. You should never force the plot by hastening the process. For instance, if the origin of a character bears great relevance to the plot of the novel, you should not just mention it in passing, but describe it as it should be.
Sometimes, writers tend to force suspense onto their readers. Most of the times, the "suspense" is usually an incomplete plot or novel. You should not force suspense or mistake it for incomplete work. As a writer, if you are not well versed in creating suspense, you should not employ it in your novel. If you must, you should do conclusive research on it.
Excessive use of characters
Yes, it is a novel. However, that does not mean that it should have an abundance of characters. Your characters should be as relevant as possible to the story. You do not have to describe the cab driver each and every time the main character takes a cab, unless the character is always driven by the same driver who also happens to have some sort of influence, direct or indirect, on the main character
Too many sub-plots
It is easy to get carried away and develop too many sub-plots within the novel. That only makes the work crowded and the plot under developed. It would be best, that if you have too many ideas for the novel, you think of how to develop it into a book series or you set them aside for another novel. You have to make the novel as easy to follow as possible. The reader needs to see the plot development from the beginning to the end without being lost in between.
At the end of the day, every mistake is something to learn from and not shy away from. Some of the mistakes can actually be used to your advantage. Every writer makes mistakes and it is better that you correct them after finishing the novel, with the consultation of an editor, instead of just leaving the novel unfinished after a few attempts.