When it comes to Seo and mistakes, many things come to mind. One thing is regarding content. Many sites make the mistake of copying exactly the same content of others. Copying unpolished content simply doesn’t work and sadly is a common mistake.
Another thing that I have seen is that many sites don't vary or diversify their anchor text. Anchor Text needs to have some variety and if it doesn't, it just doesn’t look natural.
Lastly, I often seen is poor website structure. Often sites are not laid out correctly and are not interlinked which can affect SEO to a degree.