What are common mistakes people make when setting up a custom domain on Blogspot?

Common mistakes include entering the domain name incorrectly in the Blogspot settings, failing to update the DNS records at the domain registrar with the correct values provided by Blogspot, not enabling HTTPS, forgetting to set up the 'www' CNAME record, or not allowing enough time for DNS propagation. Also, ensure the domain is unlocked and transferable if you are transferring it.