404 Error on Portfolio Page or Portfolio Items are not Loading

This can happen if you have "pretty permalinks" enabled and your portfolio page has the same url as a custom post type (portfolio). WordPress won't let you do this because if you type in your portfolio url in the browser:

http://yoursite.com/portfolio

WordPress won't know if you're referring to a portfolio item or your portfolio page, and will throw a 404 error. The simple fix is to just rename your portfolio page url to something other than "portfolio"