Which button is "the middle button"? I am looking in Firefox and all three buttons under each post seem to have a border.
You don't have any actual buttons on your page. Neither <button>s or <input type="submit">, but images and divs with onclick methods. So if you can point out which "button" has the border in IE, it's probably a simple CSS matter and it should be easy for us to solve.
You could take a screenshot and circle the button in question for us.