Make sure that your chocolate hasn't begun to re-solidify before you add it to the butter. If this happens, small pieces may harden and you'll have flecks of chocolate throughout the frosting. This will make piping the icing especially difficult. If you heat the chocolate too quickly in the microwave, it could seize, giving it a grainy, rather than smooth, appearance. If this happens, toss out the chocolate and start over or your frosting will be pebbly.