I want to start off this post by saying that this piece of advice is targeted towards people who are looking to improve their artistic abilities. If you are a beginner artist that is only focused on having fun and not concerned about technical ability then this post is not for you. 

So the biggest mistake beginner artists make is that they tend to spend too much time shading and rendering a single piece of art.

Why is this a problem? Shading and rendering isn't the hardest part of learning how to draw, the hardest part is the structural elements of drawing which are the first few steps. To get good at drawing structure you have to practice it and if you spend too much time just shading, you won't have enough time to practice the early steps.

So you will see that a lot of beginner artists will spend a ton of time drawing over the course of months but wont make as much improvement as they could have if they spent more time on the structural elements. This is not to say that you can't ever shade, but stay cognizant of the fact that you need to spend time on the difficult tasks if you want to improve.

The initial drawing is the foundation of any piece of art, you can often see that advanced artists will capture so much beauty and design in their early sketches without any rendering. To get to that level of artistic ability you need to take time to practice the early stages. 

How to Avoid This Mistake?

Focus on the early stages of a drawing and generate many ideas instead of focusing on a single idea. One way to make sure you don't get caught up in shading is to draw fairly small, large drawings will make it more tempting to shade to fill out the space while smaller drawings have to be left simple. 

Try to prioritize quantity over quality in the early stages and then slowly improve the quality of the structure and style of your drawings.