When I use a Font Awesome character as the icon for a menu item, I get a box with an X in it next to the icon when I use IE 10 or IE 11. When I look at the source code, it shows a br / HTML tag inside the div that has the class for the font-awesome icon.
<div class='wp-menu-image dashicons dashicons-businessman'><br /></div>
I can cut and paste the HTML into a test HTML file, remove the br tag and that character goes away. Its presence does not impact functionality but I have a rather picky client that would not want to see that there.