Not paying enough attention to the background
Sharp backgrounds are common in documentary styles of photography and can help tell a story about the main subject. For example, in the photo below the main subject is the three men in the photo – the barber, his customer, and the man looking directly at the camera.
The detail in the background supports the main subject and helps tell its story. We can see every detail, from the wall behind the men to the barber’s tools and products. These details are an interesting and important part of the photo.
Sometimes the opposite approach is required and you need to blur the background out to remove distractions. Part of the skill of being a photographer is knowing when to blur the background and when to keep it sharp. In some portraits (like the one below, made with an aperture of f/1.8) you can use a wide aperture to blur the background and remove details that might distract from the model.
The mistake I see photographers make is not thinking about these things and taking enough care to make sure the background suits the subject.