What are some of the most common mistakes people make when putting together a portfolio

The single most common mistake is including multiple images that are too similar images that were clearly shot at the same time and only slightly different. That is a very negative indicator of a photographers ability or willingness to evaluate their own work. Just as common is loading the front of the book with decent work, and padding the back of the book with mediocre work. Changing formats vertical horizontal and sizes is another faux pas.
In the age of the digital camera with its zero cost images, most people simply shoot too much, sloppily, and edit too loosely. All photographers find it easier to shoot than to edit their own work. Shooting is fun. Taking a hard look at everything you shot and being honest with yourself about whether you have any truly outstanding images is hard. Most of us need to call on trusted colleagues to keep us honest.