Basic Mistakes Companies Make Regarding Diversity

Having no or little diversity on your website or social media. These days, people - especially younger candidates - examine a company’s online presence before even deciding to apply. There are a lot of great companies out there that devote multiple pages and posts to inherent and acquired diversity. If your company doesn’t, candidates will self-select out of applying.
All white male interview panels. This is bad for a number of reasons. (a) It sends the wrong message to female and minority candidates. (b) You’re going to get a narrower range of feedback that’s biased towards hiring more white males.
“We hire the best.” Companies love posting words to this effect all over their website. On the one hand, many well-qualified candidates suffer from Impostor Syndrome, thinking that despite their education and experience, they lack the credentials and potential for many positions. On the flip side, the Dunning Kruger effect shows that people who are incompetent in an area tend to think they are superstars. (Have you ever had a manager who thought they were Jed Bartlet but they were more Michael Scott? I have.) So the more you talk about being elite, the more likely you are to get bad candidates and scare off potential gems.
Ignoring your team’s (lack of) diversity. Managers, if you have a team that all or disproportionately white or male, and you’re not doing something about it, then you’re part of the problem. If your recruiting pipeline resulted in a homogeneous team, then it’s time for new pipelines.