When using some desktop environments such as Unity, the icon for Minecraft does not appear correctly, but is displayed as a "?" on the Launcher. I have noticed that if I toggle fullscreen a few times by pressing F11, the icon will appear normally as a crafting table, but never with fullscreen off. Other desktop environments such as GNOME, KDE, MATE, Xfce, and pretty much every desktop environment other than Unity has not been tested by me.